Temporal Behavior of Levels of the Great Lakes and Climate Variability

Transcription

1 J. Great Lakes Res. 30(1): Internat. Assoc. Great Lakes Res., 2004 Temporal Behavior of Levels of the Great Lakes and Climate Variability Stanley A. Changnon * Illinois State Water Survey 2204 Griffith Drive Champaign, Illinois ABSTRACT. Levels of Lakes Superior, Michigan-Huron, and Erie for the period were assessed to define key temporal fluctuations in their averages and extremes. Behavior of levels of Lakes Michigan-Huron and Superior has been extremely different since 1861, including vastly different longterm distributions, differences in the amount of variability over time, and differences in when their record high and low levels occurred. Record high or low 15-year events were present on one or more lakes in 64 years, and record events based on 25-year periods were present in 96 of the 141 years, both representing the presence of records during much more time than if the record events had occurred simultaneously on all lakes. These lake level differences reflect significant between-basin differences in climate conditions, and principally precipitation over time. There were two eras, and , with exceptional variability and extremes of levels on all lakes. These findings are relevant to planning for future water level conditions, for understanding recent extremes, and for considering how the sizable spatial and temporal shifts of the past could relate to future changes in the basin s climate. INDEX WORDS: Lake levels, climate change, climate conditions. INTRODUCTION The water levels of the Great Lakes remain one of the never-ending key issues in the Great Lakes basin. The levels are constantly changing, and the fluctuations of the lake levels, including the frequent setting of records for extreme levels and rates of change, have been a subject of deep concern for more than 100 years (Changnon 1990). The varied interests in the lakes over time have become more sensitive to lake level fluctuations (Horvath et al. 1988). Record high lake levels in the mid-1980s led the U.S. and Canada to conduct a major water levels reference study, considering possible ways to ameliorate the impacts of the fluctuations (Water Science and Technology Board 1989). The extensive damages from the record high levels and ensuing inundation of facilities and the erosion of shorelines and properties were estimated to range from $62 million (1992 dollars) to $72 million per year (Working Committee ). During private persons and the governments spent $102 million on shore protection facilities. * Corresponding author. Schangno@uiuc.edu Low water damages to recreational facilities were estimated at $1.2 million. Virtually every aspect of life in the Great Lakes basin is either directly or indirectly affected by fluctuating water levels (Horvath et al. 1988). Thus, those living around the lakes, and those using the lakes for various purposes including shipping, all have considerable interest, economic and/or environmental, in lake levels. Scientific understanding of climatic behavior in the Great Lakes basin evolved over the past 75 years, revealing that the climate was composed of short-term (1- to 2-year) fluctuations, mid-term oscillations (2- to 25-year), and longer-term (multidecadal-century scale) trends (Day 1926, Bruce and Rodgers 1962). In addition to the long-term climate-driven fluctuations, there are three other types of fluctuations in lake levels. There are short period fluctuations caused by weather extremes that can last from a few hours to a few days (e.g., storm surges and seiches). A second type is the regular seasonal fluctuation reflecting the annual hydroclimatic cycle from summer high levels to winter lows. The third type is the fluctuation that results from the artificial regulation of lake levels by con- 184

2 Lake Level Fluctuations and Climate Variability 185 trol works at the outlets of Lakes Superior and Ontario. The slow crustal movement upwards of the region also effectively slowly alters lake levels. The lake levels are a climate indicator reflecting both short- and long-term climatic conditions (Brinkmann 1984). Groundwater inflow to the lakes reflects historic climatic conditions, whereas streamflow into the lakes is largely a result of the climate conditions of the past 3 to 24 months. The primary climate conditions are the precipitation over the basin and the evaporation from the lakes (Brunk 1959). The amount of precipitation on the lakes and their basins is the single most important factor, followed by the amount of evaporation, mainly a wind and temperature-driven variable (COE 2001). Study of 100-year records of lake levels and precipitation in the Lake Michigan-Huron basin found a correlation coefficient of based on a 1-year lag. Thus, precipitation explains 55 percent of the variability in the lake levels (Muller et al. 1965). Evaporation rates reflect temperatures and airflow over the lakes, and represent 30 percent of the water lost from Lake Michigan-Huron (COE 1991). Four important physical conditions affecting levels are not measured and must be estimated, and these include overlake precipitation, overlake evaporation, groundwater exchange, and overland evapotranspiration (Bruce and Rodgers 1962). Evaporation, precipitation, and temperature are well measured over the land area, and runoff is measured at many but not all streams entering the lakes. Recent concerns over climate change resulting from global warming during the 21 st Century have added to the worries over future lake levels (Changnon and Glantz 1996, Changnon 1997, Quinn 1999). Will a future change bring lower or higher levels? Recent shifts in lake levels led to a major disaster-oriented assessment of the record declines in recent years and attributed these to climate change from global warming (National Geographic 2002). Many scientific studies have estimated the effects of climate change on future lake levels (Quinn et al. 1997, Croley et al. 1998). For these reasons, an assessment of the levels of the un-regulated middle lakes (Michigan-Huron and Erie) and their source of inflow (Lake Superior) was pursued to examine both short-term and midterm historic fluctuations and trends to ascertain when and where extremes were set. Lake levels are a result of several physical and anthropogenic factors with climate the dominant factor. Humans have made changes in the lakes, particularly to the channels between them, and these have also affected lake levels. Channels between Lakes Huron and Erie have been deepened during the 1930s and 1960s, leading to an estimated lowering of Lake Michigan-Huron by 37 cm over the past 70 years (Quinn 1999). Such dredging affects the ranks assigned to some of the lake level extremes. Flow through the channel between Lakes Superior and Michigan-Huron has been controlled since 1921 with subsequent changes in the amounts released in 1946 and The current outflow regulation, established in the 1977, seeks to bring the levels of Lakes Michigan-Huron and Superior to nearly the same relative position within their historic ranges of levels, and specifically to keep Lake Superior s levels between and meters (COE 1997). However, the capability to regulate the outflows from Lake Superior does not mean that a major control of the lake levels is possible. The major factors (precipitation and evaporation) can not be controlled. For example, recent lake-level fluctuations from near record high levels in 1997 to low levels by 2000 revealed that the Lake Superior outflow control had only a minimal influence on levels of Lakes Michigan and Erie (COE 2002). Other anthropogenic actions have affected levels. Since the 1870s water has been diverted away from the Lake Michigan at Chicago with a major increase in the amount diverted in the period (Changnon and Changnon 1996). This diversion is now regulated by a U.S. Supreme Court decree at 90.6 m 3 /s. Water has also been diverted into Lake Superior from Hudson Bay basin at Long Lac (began in 1941) and Ogoki (began in 1943). All these anthropogenic factors affected lake levels, although the regulation of Lake Superior has produced little bias in the levels of Lake Michigan-Huron (Quinn and Sellinger 1990). However, the diversion out at Chicago and those in at Long Lac and Ogoki have only a minor effect on water levels, being much less than the climate factors affecting lake levels (COE 2000). Recent decades have experienced record high levels and sudden severe drops in lake levels, raising two questions: 1) how many years over time actually have one or more lakes had record levels, or a near record water level, and 2) is the climate changing? Inspection of the lake levels plotted over time also suggests there have been distinct periods with more/less variability in levels, and the variation was assessed over time. A third issue addressed was the annual rate of change in lake levels, another factor seen as important in understanding lake levels. How these events reflect the region s climate

3 186 Stanley A. Changnon over time is also addressed. The findings should be useful to those planning for future lake levels. DATA AND ANALYSIS Monthly measures of lake levels for Lakes Superior, Erie, and Michigan-Huron were available from the U.S. Army Corps of engineers for 1861 to 2001, a 141-year period. These data were used to calculate 5-, 15-, and 25-year moving averages, and in turn, to compute their standard deviations. These three periods were chosen to embrace lengths of well known short-term fluctuations in continental climates. The annual mean levels (January-December) were used to compute the rate of change in levels between the average level of a given year and the average levels of the next 1- and 3-year periods. These various measures of level changes were computed for the total period of record for each lake. Climatological data assessed were historical records collected by Canadian and U.S. weather agencies, on file at the Midwestern Regional Climate Center, and the precipitation stations prior to the 1880s are less dense than those available thereafter. The selection of times (years) when extremely high or extremely low levels or variability occurred on a given lake was based on identifying high periods (or low periods) that were temporally independent without overlapping years. For example, on Lake Michigan-Huron the maximum 25-year standard deviation value occurred during , and another high but slightly lesser variable period occurred during the period. Since these periods overlapped, the lesser one ( ) was not used in the analysis to identify the top three 25- year periods of high variability on Lake Michigan- Huron during To save space, mention of Lake Huron is hereafter deleted since its levels are the same as those for Lake Michigan. AVERAGE LAKE LEVELS The 141-year temporal distributions of the levels for each lake, based on the 5-, 15-, and 25-year moving averages, serve as a meaningful base line for assessing various temporal fluctuations. The 25- year averages of Lake Superior levels indicate a long-term slow rise in lake levels from 1861 until about 1950 (Fig. 1), with no marked up or down trend thereafter. Precipitation amounts in the Lake Superior basin from 1898 to 2001 showed near average values during the period then slightly below average values through the 1930s with above average values from 1940 to 1957 (Changnon 2003). Basin precipitation was again below average from 1958 to 1972, and this was followed by slightly above average amounts from 1973 to The fluctuations in levels for the 15- year periods (Fig. 1) closely followed these basinwide precipitation fluctuations. The three sets of average levels for Lake Michigan-Huron (Fig. 2) all display a distinct U-shaped distribution for the 141-year period. However, the U-shape is less obvious in the 5-year distribution than in those for the 15- and 25-year values. Values for 15- and 25-year periods reached their all-time lows for periods ending during period. Average basin-wide precipitation values for 15-year periods were near average from 1898 to 1912, but thereafter were below average until 1956 (Changnon 2003). Amounts during the period were near average and then became much above average from 1973 to This precipitation distribution agrees well with the 15-year lake level fluctuations (Fig. 2). The Lake Erie levels had 141-year distributions (Fig. 3), based on 25-year averages, similar to that for Lake Michigan-Huron. The lowest values occurred during the period, in close agreement with the Lake Michigan distribution. However, the early period high values on Lake Erie were less than those in recent years, a notable difference from the magnitude of the early and late levels of Lake Michigan. The temporal distribution of the Lake Superior levels for 15-year periods (Fig. 4) is markedly different from those for the other two lakes. It peaks in 1907 and 1940 when the other lakes were low, and the 141-year trend is very different. These differences in the average distribution for the three lakes reveal that the climatic conditions affecting Superior s basin from 1861 until about 1950 were markedly different from those affecting Michigan- Huron and Erie. A study of precipitation over each lake s basin for the period found that the major decrease in levels of Lake Michigan- Huron during the period either was not present or was very minor in the other lake basins (Quinn and Croley 1981). A climatological study of the spatial frequency of cyclones, which produce much of the precipitation in the Great Lakes basin, showed a sharp northsouth gradient in frequencies across the basin (Angel 1996). The average annual number of cyclones over the Lake Superior basin ranges from 31 to 35, whereas the averages across southern Lake

4 Lake Level Fluctuations and Climate Variability 187 FIG. 1. Average lake level curves based on the 5-, 15-, and 25-year moving averages of the levels of Lake Superior for Michigan and Lake Erie range from 25 to 29 cyclones per year (Changnon et al. 1995). The upward trends of the levels of Lakes Michigan and Erie since about 1945 (Fig. 4) reflect an upward trend in annual precipitation across their basins and most of the nation since (Karl and Knight 1998). The national pattern of precipitation trends showed increases in the basins of Lakes Michigan and Erie, but no upward trend in the Lake Superior basin. This helps explain the lack of a comparable increase in levels of Lake Superior. The regulation of Lake Superior levels also tended to maintain levels in the upper portion of the distribution (Quinn 1978).These precipitation increases have led to marked increases from 1948 to 1998 in annual streamflows in Wisconsin, lower Michigan, and Ohio where rivers and streams feed Lakes Michigan and Erie (Lettenmaier et al. 1994). Their analysis of national temperature trends showed significant increases since 1948 in northern Wisconsin and Minnesota, including the basin of Lake Superior, and this may relate to an increase in lake evaporation, another factor limiting any increase in the lake s levels. The lower Michigan and Ohio area had significant temperature decreases for , a condition which would likely reduce evaporation over time and also lead to increased lake levels. One analysis of the fluctuations in levels of the lakes claimed a distinct basin-wide change in climate occurred around 1970 (Hartmann

5 188 Stanley A. Changnon FIG. 2. Average lake level curves based on the 5-, 15-, and 25-year moving averages of the levels of Lakes Michigan-Huron for ), whereas another assessment claimed the period had two distinct precipitation patterns: one classed as below average (basin mean of 78.2 cm) for , and one with above average amounts (mean of 83.2 cm) for the period (Great Lakes Commission 1986). RECORD HIGH AND LOW LEVELS The ending years of the three highest and three lowest levels for 5-year periods attained on each lake during appear on Table 1. The values show considerable temporal grouping of the high and low periods. For example, two lakes had high levels in the early 1970s, ranked as the second highest of the 141-year period. All three lakes had a peak again during the early 1980s. However, four of the nine highest values were isolated in time and on a single lake. For example, only Lake Michigan had a high during , and only Lake Superior had a high during Timing of the low periods on Lakes Michigan and Erie were in close agreement, all ending in 1927, , and This reflects the strong impact of the outflow from Michigan to Erie, as well as similarity in their climate conditions (Brinkmann 1983a). Two of the low-level periods

6 Lake Level Fluctuations and Climate Variability 189 FIG. 3. Average lake level curves based on the 5-, 15-, and 25-year moving averages of the levels of Lake Erie for on Lake Superior occurred during the period, much earlier than those on the other two lakes. This outcome reveals that weather conditions capable of producing extremely dry conditions over relatively short periods, such as five years or less, often exhibit considerable spatial continuity across the basins of Lakes Erie and Michigan. But, these anomalous conditions seldom extend over the Lake Superior basin, and those over the Lake Superior basin often do not extend over Lake Michigan (Brinkmann 1983b). The years of occurrence of the first and second ranked high 15-year levels, and the first and second ranked low levels for 15-year periods for the three lakes are shown in Table 2. The impact of the high and low 5-year levels (Table 1) is evident in the 15- year outcomes. Lakes Michigan and Erie both had low 15-year periods ending in and in 1969, and both periods embraced the low 5-year periods ending in 1936 and 1966 (Table 1). Analysis based solely on the record highest and lowest 15-year periods reveals that 77 years during were without a record on one of the three lakes, whereas 64 years (45 percent of the total) experienced a record event on at least one of the three lakes. If the three record highs and lows had all occurred at the same time, as occurred in for Lakes Michigan and Erie (Table 2),

7 190 Stanley A. Changnon FIG. 4. Moving average lake levels based on 15-year periods. records would have occurred in only 30 of the 141 years, not in the 64 years with records. In essence, several extreme lake levels were occurring quite independently such as a record low on one lake and none on the other two, leading to many more years than expected with record levels. An analysis of the extreme lake levels based on values for the 25-year periods resulted in the periods shown in Table 3. During the period, there were 45 years when no record level was occurring, but a record high and/or low occurred in 96 of the 141 years, or 69% of the total time. If there had been timing agreement of the three highest and three lowest periods of all three lakes, only 50 years would have experienced a record. The main temporal overlap in record periods was in the low periods on Lakes Michigan and Erie (both had lows during ), and the high on Lake Superior overlapped them from 1928 to The findings showing the periods when record, or near record, high and low lake levels occurred, based on 15- and 25-year periods, reveal that considerable spatial difference frequently existed between the lakes with record highs on one lake when record lows occurred on another lake. This reveals that major spatial variations in both precipitation and evaporation exist across the three basins for such multi-year periods. The results also reveal that the influence of these interconnected lakes on their levels is not always large. Record levels (high or

8 Lake Level Fluctuations and Climate Variability 191 TABLE 1. Five-year periods when extremely high and extremely low lake levels occurred on Lakes Michigan, Erie, and Superior. These include the three highest and three lowest ranked values during Shown for each period is the lake (M = Michigan, S = Superior, E = Erie), and the rank of three periods with 1 = the most extreme and 3 = the third ranked extreme value. Ending year of highest Ending year of lowest three 5-yr value three 5-yr values 1878 (M-2) 1869 (S-1) 1887 (M-1) 1893 (S-3) 1954 (S-3) 1926 (S-2) 1975 (S-2) 1927 (M-3, E-2) 1976 (E-2) 1936 (E-1) 1987 (S-1, M-3, E-1) 1937 (M-1) 1998 (E-3) 1966 (M-2, E-3) TABLE 2. The years when the two highest ranked lake levels and two lowest ranked lake levels occurred, based on 15-year periods. Highest two levels 1 Lowest two levels M S E S S E S M E-1, M M-2, E-2 1 M = Michigan, S = Superior, E = Erie; 1 = rank one, 2 = rank two highest level. low) for 15-year periods were present in 45 percent of all years sampled and in 69 percent of the years with 25-year extremes. If the analysis of 15-year periods includes the two highest and two lowest independent periods on each lake during , these accounted for 66 percent of all years from 1861 to TABLE 3. The 25-year periods with the record highest and record lowest lake levels. Years of highest value Years of lowest value Michigan Superior Superior Erie Erie Michigan The general disagreement in the timing of the record high and low levels on Lakes Michigan and Superior is not surprising. The correlation coefficient between their annual basin precipitation amounts for period was only +0.62, indicating that annual precipitation on Lake Superior explained only 38% of the variations in annual precipitation over Lake Michigan basin. Assessment of 12-year wet and dry periods on the two basins, defined as periods having at least 8 or more years with above/below average values, was used to define the occurrence of a given dry or wet condition on Lake Superior when one existed on Lake Michigan. As shown in Table 4. when Lake Michigan experienced a dry period during the era, one also existed on Lake Superior in 44 percent of the years, but in 13 percent of the dry years on Lake Michigan, there was a wet period on Lake Superior, and in 43 percent of the years neither wet or dry conditions existed in the Lake Superior basin. A stronger relationship of dry conditions existed in the later ( ) period. When wet periods existed on Lake Michigan, wet periods on Lake Superior were fewer than found for joint dry periods. For example, when wet conditions occurred on Lake Michigan, only 34 percent ( ) were wet on Lake Superior, and in the period this relationship was 39 percent. Both sets of relationships were slightly stronger in the second era, reflecting a temporal shift in regional climate conditions. TABLE 4. The frequency of 12-year wet and dry periods on Lake Superior when either condition existed on Lake Michigan for early and late eras, expressed as a percentage of the period. Dry periods on Lake Superior Lake Superior Lake Superior had no Lake Michigan had a Dry period had a Wet period Wet or Dry period era 44% 13% 43% era 64% 6% 30% Wet periods on Lake Superior Lake Superior Lake Superior had no Lake Michigan had a Dry period had a Wet period Wet or Dry period era 25% 34% 41% era 30% 39% 31%

9 192 Stanley A. Changnon TABLE 5. The three 5-year periods on each lake with the greatest variability in the lake levels. Rank Lake Superior Lake Michigan Lake Erie VARIABILITY OF LAKE LEVELS OVER TIME The standard deviations (SD) of the lake levels were calculated for the moving averages of each of the three periods (5, 15, and 25 years). These SD values were used as a measure of the temporal variability in lake levels between 1861 and The standard deviation values on Lake Michigan for 5-year periods were the highest, and those of Lake Superior were the lowest throughout the 141- year period. The magnitude of the fluctuations in the SD values of Lake Superior did not shift during the 141-year period of study. The lack of an increase since the 1920s, as found on the other lakes, may reflect the regulation of Superior s outflows that began in Lake Erie s SD fluctuations from 1861 until the 1920s were different from those of Lake Michigan. Thereafter, the timing of the highs and lows in SD values on Lake Erie were similar to those on Lake Michigan, and levels of both lakes had greater variability after All three lakes experienced four major highs in their 5-year SD values. The one during exhibited the greatest variability on Superior but the variability peaked during period on the other two lakes. The periods with extreme variability in lake levels for 5-year periods are listed in Table 5. Levels on all three lakes exhibited extreme variability during the era, a time when the nation s major droughts of the 1930s began. All three lakes also had high variability values for 5- year periods during the era. The other three periods of high variability occurred as timeisolated events on only one lake. The SD values calculated for the 15-year moving averages of the three lakes from 1861 to 2001 (Fig. 5) reveal several between-lake differences. First, the 141-year variability of levels of Lake Superior with SD s of 0.45 to 0.7 was notably less than those for Lakes Michigan-Huron and Erie with SD s of 0.6 to 1.5. Regulation of Lake Superior s outflow to Lake Michigan began at minor level in 1902, then was altered in 1921 to a control plan established in 1916, an endeavor not closely followed until 1941 when a new control plan was issued (COE 1993). From 1941 to present, different regulatory rules have been in place and closely followed, and a regulation plan of 1977 sought to deal specifically with high lake levels and to balance levels between Lake Michigan-Huron and Lake Superior. Comparison of the SD values for Lake Superior before 1921 and those after 1941 does not suggest changes in the degree of variability with time, either before or after the different regulations. A second finding derived from Figure 5 is that the variability on Lake Erie had a similar time distribution as did the values for Lake Michigan, but the Lake Erie SD values were mostly lower. Variability of levels of both lakes peaked in the era, whereas the peak SD value on Lake Superior was during The 15-year SD values of Lakes Michigan and Erie generally increased over time from 1861 to 1975, and thereafter became lower. The SD values for Lake Superior showed no up trend and were essentially flat for 141 years. Variability values of Lakes Superior and Michigan were similar during when both basins had near average precipitation (Changnon 2003), but the magnitude and type of fluctuations differed in all other years. Figure 6 presents the ranges in precipitation values, expressed as percent of average, for seven discrete 15-year periods on the basins of Lakes Superior and Michigan for the period (Changnon 2003). The ranges shown are based on the highest and lowest values from the numerous climate districts in each basin. Comparison of the curves of the two lakes reveals two important findings relevant to the SD values of the lake levels. First, comparison of the two lakes sets of curves clearly illustrates that Lake Superior experienced much less variability of precipitation in all but one of seven 15-year periods, thus agreeing with findings exhibited on Figure 5. Second, the variability of precipitation amounts on the Lake Michigan basin began a major increase after 1942 that easily outmatched that on Lake Superior. Variability of precipitation on the Lake Superior basin raised from that during the droughts of the period to , but the variability was small and as shown on Figure 6, did not change much in the following three 15-year periods. This could reflect regulation endeavors. Comparison of the SD values based on the 25- year averages showed Lake Michigan s values were highest when Superior s values were lowest. Fur-

10 Lake Level Fluctuations and Climate Variability 193 FIG. 5. Curves of the variability in lake levels for 15-year periods on all lakes. thermore, the general long-term trends of the three lakes differed. SD values for Lakes Michigan and Erie exhibited an upward trend from 1861 to 1988, whereas the trend of the 141-year SD values on Lake Superior was essentially unchanging with time. The distributions of 25-year variability values on Lakes Michigan and Superior were unlike for the period. For example, Lake Superior had a peak in SD values during the period and this was a low variability period on Lake Michigan. Lake Michigan s all-time peak occurred during , whereas that on Lake Erie occurred during The SD values for Lakes Michigan and Erie dropped rapidly after The times when periods of high variability of lake levels occurred were assessed for each lake and findings intercompared for the three durations (5, 15, and 25 years). The three periods on each lake with the highest SD values were identified and those selected had to be independent in time with no overlap in years. The three periods with the highest variability in lake levels for the 15-year and 25-year periods appear on Figure 7. Four periods are shown for Lake Erie since the third and fourth ranked periods had

11 194 Stanley A. Changnon FIG. 6. Range of precipitation values, expressed as a percent of average, on the basins of Lakes Michigan and Superior for seven 15-year periods during essentially equal values. The top variability for 25- year periods shows that all three lakes were experiencing high variability during three time periods: , , and The ranks of the 15-year period values displayed on Figure 7 show little temporal agreement. For example, each of the three lakes had highly variable 15-year periods ending during the era, but their ranks were #1 on Superior, #2 on Erie, and #3 on Michigan. Inspection of the 25-year variability periods reveals Lakes Michigan and Erie experienced their greatest variability during the late 1960s to mid-1980s, but Lake Superior s variability in this same period was only ranked third highest during the 141-year period. MAJOR LEVEL CHANGES DURING 1- AND 3-YEAR PERIODS The average annual levels on each lake were compared with the averages occurring during the next year and ensuing 3 years. The aim was to assess the major short-term shifts, up and down, in lake levels. Relatively rapid major changes in levels often create major adjustment problems for many lake activities (Horvath et al. 1988). The shifts in levels between individual years on Lake Michigan are shown on Figure 8. The shift from 1930 to 1931 was a decrease to 53.3 cm below average, the greatest 1-year decrease during the period. Inspection of Figure 8 also shows other large singular 1-year increases and de-

12 Lake Level Fluctuations and Climate Variability 195 FIG. 7. The three periods with the greatest variability in lake levels during for three lakes and for 15- and 25-year periods. creases, and in general, the changes since 1920 have been greater than those in the prior 60 years. The top four rated single year increases and decreases during for the three lakes are listed in Table 6. Assessment of the years of occurrence shows that several periods of decreases extended across all three lakes including and , with Lakes Michigan and Erie experiencing a major drop (below average) in None of the years of major 1-year increases were found on all three lakes, although an increase in 1929 occurred on two lakes and one in on two lakes. These results reveal that when extremely dry conditions of 1- to 2-year durations occurred, they extended over more of the basin than did extremely wet conditions. Eight years with major increases were events found on only one lake. The years of the decreases show very few occurred before 1930 (only in 1917 on Lake Superior), and 6 of the 12 major decreases occurred during the era. The years when increases occurred showed a greater temporal distribution, occurring in the period from 1876 to The decreases for a given rank on Lake Superior were approximately 35 to 50 percent less than those on Lake Michigan. Changes in lake levels from the annual average in a given year and the average level of the ensuing 3 years also were assessed. The annual changes on Lake Michigan for 1861 to 2001 revealed an overall increase in the magnitude and frequency of extreme changes after the mid-1920s. The peak 3-year increase was 33.8 cm above the average level from 1926 to , and the greatest decrease occurred from 1929 to (Table 7). Comparison of Lake Superior s temporal distribution with that for Lake Michigan revealed major differences. The pre-1930 years on Superior had changes comparable in magnitude and frequency to those in the post-1930 period, whereas those on Lake Michigan increased after The results do not suggest that regulated outflows from Lake Superior had any influence over time on the magnitude of the major 3-year shifts in lake levels. The magnitudes of the two lakes values for 1997 to the period are not the same, with Lake Michigan having a decrease from 1997 to

13 196 Stanley A. Changnon FIG. 8. The magnitude of changes in lake levels between consecutive years for on Lake Michigan-Huron. Values are meters above or below the long-term average of 29.9 cm, double the 14.0 cm drop on Lake Superior (Table 7). Inspection of the major 3-year shifts on all three lakes (Table 7) reveals that three time periods had major shifts on all lakes. Major 3-lake decreases occurred during , , and Major 3-lake increases occurred during , , and The major feature on all three lakes was the big swing from top ranked increases in the late 1920s to large decreases in the early 1930s. SUMMARY AND CONCLUSIONS Water levels of the Great Lakes remain a major issue. The fluctuations of the lake levels, and the frequent occurrence of extreme levels and rates of change, have long been a subject of deep concern. These fluctuations largely reflect shifting climate conditions over time since climate is the major factor controlling lake levels. This assessment of the levels of four upper lakes (Superior, Michigan- Huron, and Erie) included examining short-term TABLE 6. The four greatest increases and decreases in lake levels between single years, measured in centimeters, from average lake levels, and year of occurrence, and during the period. Lake Michigan Lake Superior Lake Erie Decreases Increases Decreases Increases Decreases Increases

14 Lake Level Fluctuations and Climate Variability 197 TABLE 7. The five greatest increases and decreases in lake levels between single years and the average level of the ensuing three years, measured in centimeters, and years of occurrence, and for the period. Lake Michigan Lake Superior Lake Erie Decreases Increases Decreases Increases Decreases Increases 31.7: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : and long-term fluctuations and trends since 1860 to ascertain when and where extremes occurred and how the variability of levels has shifted over time. The 141-year average lake level distributions for 5-year and longer periods show a U-shaped temporal distribution for Lake Michigan, being highest early (19 th Century). The distribution for Lake Erie was also U-shaped but the highest values came in recent years ( ), and that for Lake Superior shows a very different distribution. It exhibited a gradual increase over time from 1861 until about 1950 and a flat trend thereafter. A review of the fluctuations in the levels of these upper lake systems offered an interesting perspective on the occurrence of record high or low lake levels. Depending on the level criteria being examined, record highs or lows were being set on one or more lakes during nearly half the time over the 141- year period. Records are set to be broken is an old adage that appears appropriate for the levels of the Great Lakes. Assessment of extremes in lake levels, high or low, shows that record extremes for 15-year periods were occurring on one or more lakes during 45 percent of the years since 1860, and 25-year records (high and/or low) prevailed on 69 percent of all 141 years. Certain periods including , , and had considerably more year-to-year variability in lake levels than did other years, a condition reflecting times of greater climate instabilities in the Great Lakes basin. Recent ( ) high levels were records for Lakes Michigan and Erie, but not for Lake Superior. Major short-term, 1- and 3-year, changes in lake levels revealed sizable differences between occurrences on the three lakes. Major differences exist in the timing of record shifts between the 1-year and 3-year changes. The considerable differences found in the behavior of lake levels during the period explain the frequent setting of record (or near record) high and low levels. This type of behavior was driven by considerable temporal and spatial variability in the regional climate conditions that control lake levels (precipitation and evaporation). The variability in the levels of Lakes Michigan and Erie was less during than during the period when it gradually increased over time. However, the magnitude of the variability in the level of Lake Superior did not change during the 141-year period. There were two periods when the levels of all three lakes exhibited great variability: and , illustrating the existence of highly variable, less stable atmospheric conditions. Both periods had a mix of exceptionally wet and dry years. A climatological interpretation of the historical fluctuations in lake levels is potentially limited by anthropogenic changes in the channels between the lakes. Alterations in the control of outflows from Lake Superior to Lake Michigan could conceivably alter record levels, the degree of variability, and rates of level changes from one year to another. However, inspection of the long-term behavior of the levels of Lake Superior does not suggest a detectable signal relating to the outflow changes. The temporal distributions of extremely high and low levels, and the variability of levels of Lake Superior do not appear to reflect changes in the controlled outflow since Effects of the controlled outflows on Lake Michigan levels are not evident either. For example, after the shift to strict controls of the outflows in 1941, Lake Michigan levels, as measured for 5-, 15-, and 25-year periods, did not show post-1941 perturbations in levels. Furthermore, 1941 was followed by ever increasing levels (but not reaching the high levels of the 19 th Century). What do these comparisons of averages, extremes, and variability of levels of the four lakes reveal about the climate conditions of their basins

15 198 Stanley A. Changnon over the past 141 years? Recall, of course, that two of the critically important conditions affecting lake levels go unmeasured including overlake precipitation and evaporation, and their impact on levels is best revealed in how the levels behave. The comparisons of the levels of the lakes revealed two important findings about the past climate conditions that control lake levels. First is the strong evidence of two major periods of change in the climate that significantly affected levels of all four lakes. One period occurred during the era: The variability of the levels of all lakes was exceptionally high during these 16 years. The levels reached lows in , rose rapidly ( ), fell rapidly ( ), and reached record lowest levels in the 1930s. After this period the levels of three lakes increased (but not Superior). After this period the variability of three lakes was much greater than in prior years (but not Superior). The second period of instability occurred during the period: The variability of levels on all four lakes was exceptionally large (in and ), but did not match values during the era. All four lakes had record high, or near record highs twice during with the highest period on all lakes. Lake levels oscillated, descending rapidly twice: during and (greatest fall during the 141-year period). Other studies have noted there had been some form of climate change on the basin during the 141- year period. One was reported to have occurred around 1970 (Hartmann 1988), and the Great Lakes Commission (1986) noted a change in annual precipitation around Another study that compared precipitation on the basins of Lakes Superior and Michigan-Huron concluded that a major change had occurred during the late 1920s (Changnon et al. 1990). These various observations had identified one of the two periods of change noted herein. The second important climate-related finding is the clear evidence of major spatial differences in the climates of the basins of the lakes. Climate conditions controlling lake levels on Lake Michigan- Huron are quite different than those controlling levels of Lake Superior: Their average height distributions for are totally different. Times of most of their near record highs and lows differ, particularly for the high levels, although agreement was reached in the two exceptional climate periods ( , and ). The variability of levels on Lake Michigan- Huron (for all durations) is much higher than that for Lake Superior levels, with the variability of Michigan levels increasing steadily after 1936 but not the Superior levels. These differences in the behavior of the lake levels and hence climate conditions, have been noted by others. Brinkmann (1983b) found a distinct climate difference between the two basins, and another study found major temporal disagreements in the incidence of 12-year wet and dry periods (Changnon et al. 1990). The correlation coefficient between the precipitation of the two lakes is only Angel (1996) found differences in the temporal distribution of precipitation-producing cyclones with trends upward over time on Michigan-Huron and Erie basins, but no up or down trends in cyclone frequency over the Lake Superior basin. Due to the large influence of the outflow from Lakes Michigan-Huron on the levels of Lake Erie, a good interaction is expected and many aspects of the levels of Lakes Michigan-Huron and Erie were alike. This included their average long-term distributions (flat then increasing after 1935), their joint increases in variability of levels after 1935, and in the times of occurrence of their record high and low levels. However, the levels of the two lakes disagreed in three ways. The highest levels on Michigan-Huron were in the period but the highest on Erie came during This is partly a result of the dredging of the channels between the two lakes (Quinn 1999). Furthermore, the high variability of levels and the highest levels attained for 25-year moving average periods were not in agreement. Examination of the extremes and fluctuations in the lake levels for 5-year periods showed more agreement amongst the lakes than found in their levels for longer, 15- and 25-year periods. This reveals that climate conditions affecting the lake levels, precipitation and evaporation, are more

16 Lake Level Fluctuations and Climate Variability 199 frequently similar across all four basins for shortterm periods, such as 5 years, but the conditions differ much more when sampled over 15-year or longer periods. The period of great variability and record low lake levels was followed by a shift in the climate on three lakes (Michigan-Huron and Erie). This has been noted to have been an ever wetter period (Karl and Knight 1998) and a cooler period (Lettenmaier et al. 1994) over their basins, conditions conducive to increased lake levels. On Lake Superior s basin the temporal precipitation increase did not occur and temperatures rose over the last four decades, conditions that would not result in lake level increases as occurred on the other lakes. The key question relates to what type of climate will follow the unstable conditions of ? The results should have considerable value for those planning for future water level episodes. Those attempting to understand recent major shifts in lake levels can view these conditions from a long historical perspective based on climate aberrations sampled over 140 years. The findings also reveal major climate fluctuations, both in space across the basin and over time, factors very relevant to considering the dimensions of potential future climate changes and their effects on lake levels. ACKNOWLEDGMENTS The lake level data were provided by the U.S. Army Corps of Engineers Office in Detroit, and I appreciate their assistance. Jon Burroughs prepared the graphics. REFERENCES Angel, J.R Cyclone Climatology of the Great Lakes. Illinois State Water Survey, Champaign, IL, Miscellaneous Pub Brinkmann, W.A.R. 1983a. Secular variations of surface temperature and precipitation patterns over the Great Lakes region. J. Climatol. 3: b. Association between net basin supplies to lake Superior and supplies to the lower Great Lakes. J. Great Lakes Res. 9: Association of summer temperature and precipitation patterns over the Great Lakes region and water supplies in the Great Lakes. J. Climat. 4: Bruce, J.P., and Rodgers, G.K Water Balance of the Great Lakes. GLI7, Great lakes Institute, University of Toronto, Toronto. Brunk, I Precipitation and the levels of Lakes Michigan and Huron. J. Geophys. Res. 64: Changnon, D., Noel, J., and Maze, L Determining the average cyclone frequencies using equal-area circles. Monthly Weather Review 123: Changnon, S.A Great Lakes waters: Too little or too much? In Proc.Supplying Water and Saving the Environment for Six Billion People, pp , Amer. Soc. Civil Eng., Washington, D.C Climate change and the water levels of the Great Lakes, Shore and Beach 65: Temporal Distribution of Midwestern Precipitation During the 20 th Century. Illinois State Water Survey, Champaign, IL., and Changnon, J.M History of the Chicago diversion and future implications. J. Great Lakes Res. 22: , and Glantz, M.L The Great Lakes diversion at Chicago and its implications for climate change. Climatic Change 32: , Leffler, S., and Shealy, R Effects of Past Low Lake Levels and Future Climate-related Low Lake Levels on Lake Michigan, Chicago, and the Illinois Shoreline. Illinois State Water Survey, Champaign, IL, Report of Investigation 110. Corps of Engineers (COE) Precipitation on the Great Lakes. Great Lakes Update 74: The regulation of the outflow from Lake Superior. Great Lakes Update 101: High Water Level Concerns. //sparky.nce. usace.army.mil/levels Water level controls. Great Lakes Update 139: Current Great Lakes studies. Great Lakes Update 145: Are Great Lakes Water Levels Recovering? Great Lakes Update 148:1 4. Croley, T., Quinn, F., Kunkel, K., and Changnon, S.A Great Lakes hydrology under a transposed climate. Climatic Change 38: Day, P.C Precipitation on the drainage area of the Great Lakes. Monthly Weather Review 54: Great Lakes Commission Water Level Changes. Ann Arbor, MI. Hartmann, H Potential Variations in Great Lakes Water levels: A Hydrologic Response Analysis. GLERL, Ann Arbor, MI. NOAA Technical Memorandum ERL 68. Horton, R.E., and Grunsky, C Hydrology of the Great Lakes. Part 3, Appendix 2, Chapter 7. Report of the Engineering Board of Review of the Sanitary District of Chicago, Chicago. Horvath, F.J., Jannereth, M.R., and Shafer, C Impacts of Water Level Fluctuations. Land and Water Management Div., Michigan Dept. of Nat. Resources, Lansing, MI.

17 200 Stanley A. Changnon Karl, T.R., and Knight, R.W Secular trends in precipitation amount, frequency and intensity in the U.S.A., Bulletin Amer. Meteoro. Soc. 79: Lettenmaier, D.P., Wood, E., and Wallis, J Hydro-climatological trends in the continental U.S., , Journal Climatol. 7: Muller, F.B., Gervais, J., and Shaw, R The Effect of Basin Precipitation on the Level of Lake Michigan/Huron. Canada Department of Transport, Toronto, Circular 4264 Meteorological Branch. National Geographic Down the drain? The incredible shrinking Great Lakes. Quinn, F.H. 1978: Lake Superior regulation effects. Water Resources Bulletin 14: Anthropogenic changes to Great Lakes water levels. Great Lakes Update 136:1 4., and Croley, T.E The role of precipitation climatology in the hydrologic design and planning on the Laurentian Great Lakes. In Preprints 4 th Conference on Hydrometeorology, pp. 7 11, American Meteorological Society, Boston., and Sellinger, C.E Lake Michigan record levels of 1838, A present perspective. J. Great Lakes Res. 16: , Croley, T., Kunkel, K., and Changnon, S Laurentian Great Lakes hydrology and lake levels under the transposed 1993 river flood climate. J. Great Lakes Res. 23: Water Science and Technology Board Great Lakes Water Levels: Shoreline Dilemmas. National Research Council, National Academy Press, Washington, D.C. Working Committee Land Use and Management. Annex 2, Great Lakes-St. Lawrence River Basin Levels Reference Study, Toronto, Canada. Submitted: 28 May 2003 Accepted: 9 December 2003 Editorial handling: Barry M. Lesht