Analysis of Vermillion River Stream Flow Data (Dakota and Scott Counties, Minnesota)

Transcription

1 ST. ANTHONY FALLS LABORATORY Engineering, Environmental and Geophysical Fluid Dynamics Project Report No. 54 Analysis of Vermillion River Stream Flow Data (Dakota and Scott Counties, Minnesota) by William Herb and Heinz Stefan Prepared for Minnesota Pollution Control Agency St. Paul, Minnesota July 2008 Minneapolis, Minnesota

2 The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, religion, color, sex, national origin, handicap, age or veteran status. 2

3 Abstract As part of an effort to characterize the response of the Vermillion River to surface runoff, the flow records from seven gaging stations were analyzed to determine the data quality, typical low flows, and the contribution of major tributaries to the total flow. The flow record from the U.S. Geological Survey gaging station at Empire appears to give the most reliable flow data and has the longest record (33 years). An effort by the Minnesota Department of Natural Resources (MNDNR) to recalibrate other Vermillion River flow gaging stations appears to have resulted in self-consistent data for 2007 flows in the main stem and at tributary stations. Although many of the stream gaging stations examined in this study have flow records of up to eight years, flow data prior to 2007 have to be used with caution. To select a representative summer low flow for a stream temperature analysis, the 33 year record at the USGS station at Empire was analyzed to determine 7Q2, and 7Q0 low flows, and monthly median and mean flows. The monthly and 7Q2 flows are representative of typical flow conditions, while the 7Q0 flow is an extreme low flow condition. Analyses were made both with and without the flow contribution of the Empire WWTP (wastewater treatment plant), to asses both pre- and post-2007 conditions. For a July/August composite average, the mean, median, 7Q2, and 7Q0 flows at the USGS station are 68.0, 43.4, 28.6, and 2.5 cfs, respectively, without the WWTP effluent. Mid-summer (July/August) flow conditions at the USGS station were further characterized by histograms of daily flows at the USGS stream gaging station for the years 982 through With the WWTP effluent included daily July/August flows in the Vermillion River occurred in the range from 20 to 60 cfs for more than 6 days in each month; without the WWTP effluent the flows range from 0 to 50 cfs. The Vermillion River is a coldwater stream because a substantial portion of its flow is from groundwater (base flow). To estimate the base flow fraction in the Vermillion River, the original USGS flow data were processed using the USGS PART program. For the entire record, the overall base flow was estimated to be 84% of the total stream flow. The base flow fraction varied from 72% in March to 98% in January. South Creek, North Creek, and South Branch contribute significant flows to the Vermillion River main stem, but the fraction of flow attributable to each tributary varies from year to year. Probable causes of the observed year to year variation are () differences in the spatial patterns of rainfall from year to year and (2) issues with the gaging sites. For the years 200, 2006 and 2007 combined, South Creek supplied about 5% of the flow at the USGS station. North Creek and Middle Creek supplied about 24% of the flow at the USGS station. For the most recent 4 years ( ) the flow in South Branch was about 4% of the flow at the USGS station. As part of the flow study, precipitation data for the Vermillion River watershed were also examined. Both the University of Minnesota Rosemount Experiment Station and the Empire WWTP appear to have reliable, sub-hourly rainfall data for the watershed. Daily rainfall totals for the two stations were fairly well correlated (R 2 = ). 3

4 Table of Contents. Introduction Daily streamflows at the USGS stream gaging station near Empire, MN Streamflows along the Vermillion River and in tributaries Flows at Annual Time Scales Flows at Monthly Time Scales Flows at Daily Time Scales Analysis of precipitation data in the Vermillion River watershed Conclusions Acknowledgments References

5 . Introduction Streamflow and stream temperature are important parameters that determine aquatic habitat quality in coldwater stream systems. Summer low flow conditions are of particular concern, since low flow volumes respond more strongly to external heat inputs from atmospheric heat transfer and surface runoff. The ability of coldwater streams to support trout populations through summer low flow periods often depends on groundwater inputs. This report summarizes results of an effort to characterize flow conditions in the Vermillion River and its tributaries, upstream of the town of Vermillion, MN (Figure.). The Vermillion River is a MNDNR-designated trout stream south of the Twin Cities metropolitan area in Dakota and Scott Counties. Urban development in the Vermillion River watershed is common. The primary purpose of the analyses described in this report was to characterize stream flow conditions that can serve as initial conditions for subsequent studies of stream temperatures and surface water inflows under past and future watershed conditions. A secondary purpose was to provide information on the quality of observed flow data. Flow analyses were primarily made at monthly time scales to determine seasonal flow regimes and water budgets, including estimates of the distribution of groundwater inflows to the main stem of the Vermillion River and its tributaries. Flow data used were from one U.S. Geological Survey (USGS) stream gaging station and eight gaging stations maintained by the Dakota County Soil and Water Conservation District (SWCD). The 33-year record of the USGS gaging station enabled a low flow recurrence analysis, while the shorter records of the other flow gaging stations were used to estimate the typical distribution of flow inputs from the major tributaries. Overall, the 2007 flow records appeared to be more reliable than those from previous years. The information on streamflow in this report is also an extension of a baseflow and groundwater recharge analysis (Erickson and Stefan 2008) that used only the data from the USGS gaging station at Empire, and was conducted using an annual timescale. 5

6 Figure.. The Vermillion River watershed. The study area includes the Vermillion River main stem and major tributaries upstream of the VR803 station, near the town of Vermillion, MN. 6

7 2. Daily streamflows at the USGS stream gaging station near Empire, MN Daily streamflow data for the Vermillion River are available online ( for the USGS stream gaging station near Empire for the period 975 through 2007 (Figure 2.). The data were analyzed to determine mean and median daily flow rates, and 7-day low flows with 2 and 0 year recurrence intervals (7Q2 and 7Q0). The 7Q2 and 7Q0 low flows were determined using a Pearson Type III distribution, using procedures given by Martin et al.(999). The analysis was made for the entire record, seasonal data and monthly data. The results are summarized in Table 2. and Figure 2.2. Monthly 7Q2 flows varied from 3.7 cfs (February) to 68.8 cfs (April), while 7Q0 flows varied from 7. cfs (January) to 34.7 cfs (April). For most of the flow record, the Empire wastewater treatment plant (WWTP), operated by the Metropolitan Council Environmental Services (MCES), discharged into the Vermillion River about 4 miles upstream of the USGS stream gaging station. This effluent of domestic wastewater is independent of hydrologic processes (rainfall, infiltration, surface runoff, evapotranspiration) in the Vermillion River watershed, and since 2007 has been diverted through a pipeline directly to the Mississippi River. To assess future flow regimes in the Vermillion River without the WWTP effluent, the flow data record at the USGS station was modified. The daily WWTP effluent rate was assumed to equal the inflow rate. Daily inflow rates to the Empire WWTP were available from the MCES for the period In response to progressive urbanization the WWTP inflow rates.increased from about 5 cfs in 982 to about 3 cfs in 2006 (Figure 2.3), with an overall average of 0.7 cfs for the period. With increasing WWTP discharges the low flow statistics of the Vermillion River at the USGS gaging station near Empire changed also. To account for this change the flow statistics presented in Table 2. for the period were recalculated and are presented in Table 2.2 for the period The flows in Table 2.2 are higher than in Table 2.. An adjusted daily flow rate was then calculated for the USGS gaging station near Empire, by subtracting the estimated WWTP effluent from the reported USGS flow data. The adjusted USGS flow is given in Figure 2.3. It represents the response of the Vermillion River flow to natural hydrologic events in the watershed without the input from a WWTP. Present and future flows in the Vermillion River do not include the WWTP effluent either. The flow statistics of the adjusted flow are given in Table 2.3. The seasonal variation of mean monthly flow, 7Q2, and 7Q0 flows calculated using the adjusted USGS flow data are compared to the unadjusted flows in Figure 2.4. On average, the adjusted monthly mean, 7Q2, and 7Q0 flows were 20%, 0%, and 20% lower, respectively, compared to the unadjusted flows. The mean monthly flows include large peak flows due to surface runoff. To better characterize typical mid-summer flow conditions at the USGS station, histograms of daily flow data were made for the period July to August 3, for the years 982 through 2005 (Figure 2.5). (Since the results of this study are to be used in thermal impact studies, June was not included, because ) water temperatures have not peaked yet, and 2) streamflows are still relatively high) With the WWTP effluent (for the original USGS flow data), daily July/August flows occur in the range from 20 to 60 cfs for more than 6 days in each month; without the WWTP effluent (for the adjusted USGS flow data), the flows range from 0 to 50 cfs. 7

8 Calculated median flows for each month, summer periods, and the entire year are presented in Tables 2. to 2.3. The median July/August flows for the period are 55 and 43 cfs, respectively, for the original and adjusted USGS stream flow data. The Vermillion River is a coldwater stream because a substantial portion of its flow is from groundwater (base flow). To estimate the base flow fraction in the Vermillion River, the original USGS flow data were processed using the USGS PART program, a simple baseflow separation program developed by the USGS ( When the program was applied to the entire record, , the overall, base flow was estimated to be 84% of the total streamflow. The base flow fraction varied from 72% in March to 98% in January (Table 2. and Figure 2.6). The overall mean of 84% is in agreement with the results of Erickson and Stefan (2008). Analysis of the adjusted USGS data (USGS record -WWTP effluent) yielded an overall base flow fraction of 80% (Table 2.2). Table 2.. Mean, median, 7Q2, and 7Q0 flows and baseflow fraction in the Vermillion River at the USGS gaging station near Empire for annual, summer, and monthly time periods, Month Mean (cfs) Standard Dev. (cfs) Median (cfs) 7Q2 (cfs) 7Q0 (cfs) Baseflow Fraction (%) Jan-Dec May-Sept Jul-Aug Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec

9 Table 2.2. Mean, median, 7Q2, and 7Q0 flows for the Vermillion River at Empire for annual, summer, and monthly time periods, Month Mean (cfs) Standard Dev. (cfs) Median (cfs) 7Q2 (cfs) 7Q0 (cfs) Jan-Dec May-Sept Jul-Aug Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Table 2.3. Mean, median, 7Q2, and 7Q0 flows and baseflow fraction for the adjusted USGS station at Empire (USGS Empire WWTP effluent), for annual, summer, and monthly time periods, Month Mean (cfs) Standard Dev. (cfs) Median (cfs) 7Q2 (cfs) 7Q0 (cfs) Baseflow Fraction (%) Jan-Dec May-Sept Jul-Aug Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec

10 00 USGS flow station Figure 2.. Daily flow in the Vermillion River at the USGS gaging station near Empire, Minnesota, USGS flow station 50 Mean +- SD 7Q2 low flow 7Q0 low flow 50 0 Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Figure 2.2. Monthly mean, 7Q2, and 7Q0 flows in the Vermillion River at the USGS station near Empire,

11 00 USGS - Empire Effluent Empire Effluent Figure 2.3. Estimated daily effluent flow rate from the Empire WWTP and the daily flow at the USGS station without the WWTP effluent, USGS flow station Mean Median 7Q2 low flow 7Q0 low flow Mean, w/o WWTP Median, w/o WWTP 7Q2, w/o WWTP 7Q0, w/o WWTP 50 0 Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Figure 2.4. Monthly mean, median, 7Q2, and 7Q0 flows in the Vermillion River at the USGS station near Empire, with and without the treatment plant effluent, for

12 Number of Days per Month USGS Flow w/o Empire WWTP Effluent July - August, Median flow = 43 cfs Number of Days per Month USGS Flow w/ Empire WWTP Effluent July - August, Median flow = 55 cfs Figure 2.5. Histogram of daily flows in the Vermillion River at the USGS station near Empire, , without (top) and with (bottom) discharge from the Empire WWTP. The x-axis labels give the lower end of each 0 cfs bin range. Daily flows in the range of cfs are not shown. 2

13 40 20 USGS flow station ; % Baseflow Streamflow Baseflow % Baseflow Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Figure 2.6. Monthly mean streamflow, baseflow, and baseflow fraction for the Vermillion River at the USGS station near Empire (original data). 3. Streamflows along the Vermillion River main stem and tributaries Daily streamflow data from nine other sites on the Vermillion River (Figure 3. and Table 3.) were obtained from the Dakota County SWCD and analyzed to estimate the fraction of total flow coming from tributaries and from direct groundwater input to the main stem. These data included a recent recalibration of the rating curves by the Minnesota DNR. Two sites were installed in 2007 (CED3/LKV2 and VR62), and therefore have short records, while the other seven sites have 8-year records. Flow at the VR809 station is very intermittent, with, on average, 60 days of measureable flow per year. Flow data for the remaining 6 gaging sites (SC804, VR807, MC80, USGS, SB802, VR803) were examined at annual, monthly and daily time scales to ) evaluate the quality of the data and 2) determine the typical contributions of flow from the major tributaries and other sources. 3. Flows at Annual Time Scales Mean annual flows (April October) for the seven gaging sites are given in Table 3.2. The mean annual flows vary significantly from year to year, e.g. at the USGS stream gaging station mean annual flows vary from 33 cfs in 2002 to 63.9 cfs in The gaging stations for tributaries and upstream areas of the main stem show higher relative variations than the USGS station, e.g. 8.6 to 52 cfs at SC804 and 0.4 to 58.9 cfs at SB had the highest annual average flow 3

14 at all stations except SB802, which had the highest flow in The ratio of mean annual flow at each gaging site to the corresponding flow at the USGS station is given in Table 3.3. In 2004 and 2005 the mean annual flow at VR807 (Main stem plus South Creek) is a high fraction (88% and 69%, respectively) of the flow (unadjusted) at the USGS station. The flow ratios for stations SB802 (South Branch) and VR803 (Main stem) are also given for comparison in Table 3.3, although they are downstream of the USGS station. The flow ratio for SB802 (South Branch) is significantly higher (30-55%) prior to 2004 than after 2004 (6-2%). Overall, the flow ratio for each station varies significantly from year to year, suggesting that the contribution of each portion of the watershed varies from year to year. 3.2 Flows at Monthly Time Scales Mean monthly flows were calculated from the daily flow data for the flow gaging stations and the Empire WWTP effluent. Table 3.4 summarizes mean monthly flows at six flow gaging sites (SC804, VR807, MC80, USGS, SB802, VR803), the Empire WWTP effluent, and three derived flows: South Creek = (VR807 SC804), GW = USGS - (VR807+MB80+Empire), and GW2 = VR803 - (USGS+SB802). GW and GW2 are intended to estimate groundwater inflows to the main stem between VR807 and the USGS station (GW) and from the USGS station to VR803 (GW2), but also include flows from minor tributaries. Based on the flow balances, significant groundwater flows enter the main stem both upstream and downstream of the USGS station, in addition to tributary inputs. The relationships of monthly flows at several gaging sites (Figure 3.) to the flow at the USGS station are illustrated in Figures 3.3a -3.3e. A relatively strong relationship exists between flows at SC804 (main stem) and the USGS station (Figure 3.3a); the polynomial fit implies that the flow at SC804 supplies about 3% of the flow at the USGS station for low flows, but may contribute a higher fraction in higher flow regimes, e.g. spring runoff and large storms. The relationship to flows at the USGS station is not as good for the estimated South Creek flow (VR807-SC804) if all years are considered (Figure 3.3c). Negative values of the estimated South Creek flow indicate a likely problem with either the VR807 or SC804 flow gaging station in at least month of 2000 and For the years 200, 2006 and 2007 combined, South Creek supplies about 5% of the flow at the USGS station. A relatively strong relationship exists between mean monthly flows at MC80 (North Creek plus Middle Creek) and the USGS station for the combination of all years (Figure 3.3d); the polynomial fit predicts that the combination of North Creek and Middle Creek supplies about 24% of the flow at the USGS station. Although South Branch enters the main stem downstream of the USGS station, there is a relationship between the mean monthly flows at SB802 and the USGS station (Figure 3.3e) for the most recent 4 years ( ); according to this relationship the flow in South Branch is about 4% of the flow at the USGS station. 3.3 Flows at Daily Time Scales Time series of daily flows at the four main stem gaging sites (SC804, VR807, USGS, VR803 are given in Figure 3.3a-3.3c for 2000 through Data gaps exist in the VR803 record, including July 2000 April 200, August October 2002, and April June Shorter data gaps also exist in the SC804 record during low flow periods. Some gaging issues (related to water level 4

15 recordings and/or stage discharge relationships) are apparent in the daily flow plots. In 2007, the flow peaks at the furthest downstream station on the main stem (VR803) track along with those at the other stations, particularly the USGS station. Prior to 2007, the VR803 station shows a muted response compared to the USGS station. Analysis of 2007 daily flow data shows an excellent relationship between the VR803 and USGS stations (Figure 3.4). This relationship could be used to generate daily flow information for VR803 for 2006 that is likely to be better than the VR803 gage data. Daily flow data for MC80 and NC808 are given in Figure 3.5. MC80 is just downstream of the confluence of North Creek and Middle Creek, while NC808 is on North Creek, just upstream of the confluence with Middle Creek (Figure 3.) flow data show the expected behavior, with MC80 slightly higher than NC808 due to inflow from Middle Creek. Prior to 2007, this relationship is not consistently maintained, as exemplified in Figure 3.5 for 200 and Table 3.. Summary of flow gaging sites on the Vermillion River used in this study. The USGS stream gage records data all year, while the other gages generally record data from April to October. Site Name Location Period of Record VR809 Main stem at 235 th St , intermittent CED3/LKV2 South Creek at Cedar Ave. Jun - Nov 2007 SC804 Main stem at 220 th St., upstream from South Creek confluence VR807 Main stem at Denmark Ave., downstream from South Creek confluence NC808 North Creek, upstream of confluence with Middle Creek MC80 Middle Creek, downstream of confluence with North Creek SB802 South Branch USGS Main stem at Empire VR62 Main stem at Fischer Ave., upstream of the April Nov 2007 town of Vermillion VR803 Main stem at Goodwin Ave., downstream of the town of Vermillion

16 Table 3.2. Mean flows (cfs) for April through October at seven Vermillion River gaging stations for Year SC804 VR807 NC808 MC80 SB802 USGS VR All Table 3.3. Mean flow normalized to USGS flow for April through October at six Vermillion River gaging stations for Year SC804 VR807 NC808 MC80 SB802 VR Table 3.4. Measured average monthly flows (cfs) in the Vermillion River main stem and tributaries, and calculated groundwater inputs for GW= (USGS- (VR807+MC80+Empire)) is the estimated groundwater input to the main stem from VR807 to USGS, while GW2=(VR803-(USGS+SB802) is the estimated groundwater input to the main stem from USGS to VR803. Year 2007 SC804 South Creek VR807 Middle + North Creek Empire WWTP USGS South Branch GW VR803 GW2 April May June July Aug Sept Oct Mean

17 Figure 3.. Approximate locations of the streamflow and precipitation gaging stations on the Vermillion River. The main stem includes stations VR809, SC804, VR807, USGS, VR62, and VR803. 7

18 Monthly Flow, SC804 (cfs) y = x x R 2 = Monthly Flow, USGS (cfs) Figure 3.2a. Monthly average flow for SC804 vs. USGS, The polynomial fit shown includes all years. 200 Monthly Flow, VR807 (cfs) y = x x R 2 = Monthly Flow, USGS (cfs) Figure 3.2b. Monthly average flow for VR807 vs. USGS, The polynomial fit shown includes and

19 Monthly Flow, South Creek (cfs) y = 8E-05x x R 2 = Monthly Flow, USGS (cfs) Figure 3.2c. Monthly average flow for South Creek vs. USGS, , where South Creek is estimated as (VR807-SC804). The polynomial fit shown includes 200, 2006 and 2007 data only. Monthly Flow, MC80 (cfs) y = 0.000x x R 2 = Monthly Flow, USGS (cfs) Figure 3.2d. Monthly average flow in North Creek plus Middle Creek (MC80) vs. flow in main stem (USGS), The polynomial fit shown includes all years. 9

20 Monthly Flow, SB802 (cfs) y = x x R 2 = Monthly Flow, USGS (cfs) Figure 3.2e. Monthly average flow in South Branch (SB802) vs. flow in main stem (USGS), The polynomial fit shown includes data from only. 20

21 SC804 VR807 0 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ SC804 VR807 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ SC804 VR807 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ Figure 3.3a. Daily stream flows at four Vermillion River main stem gaging stations in 2000, 200 and

22 SC804 VR807 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ SC804 USGS VR807 VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ SC804 VR807 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ Figure 3.3b. Daily stream flows at four Vermillion River main stem gaging stations in 2003, 2004 and

23 SC804 VR807 USGS VR / 5/ 6/ 7/ 8/ 8/3 0/ SC804 VR807 USGS VR803 4/ 5/ 6/ 7/ 8/ 8/3 0/ Figure 3.3c. Daily stream flows at four Vermillion River main stem gaging stations in 2006 and Flow, VR803 (cfs) y = x x R 2 = Flow, USGS (cfs) Figure 3.4. Relationship between daily stream flows at VR803 and the USGS gaging station, March through November,

24 0 200 NC808 MC80 0 4/ 5/ 6/ 7/ 8/ 8/3 0/ 2006 NC808 MC80 0 4/ 5/ 6/ 7/ 8/ 8/3 0/ 2007 NC808 MC80 0 4/ 5/ 6/ 7/ 8/ 8/3 0/ Figure 3.5. Daily flows at MC80 and NC808 in 200, 2006 and

25 4. Analysis of precipitation data in the Vermillion River watershed Precipitation data are available from a number of stations in and near the Vermillion River watershed. The Met Council maintains a rain gage which logs at 5 minute intervals at the Empire waste water treatment plant. The University of Minnesota maintains a climate station at the Rosemount Experiment Station which gives 30 minute rainfall data. The Lakeville airport does not consistently report precipitation data, although some readings are given in the NCDC data files for this station. The Flying Cloud airport reports precipitation reliably this station is approximately 0 miles west of Lakeville. Daily precipitation data are available for Farmington from the National Weather Service Cooperative program. Dakota County has maintained two rain gages which record at 4 minute intervals near South Branch for several years. The precipitation data assembled for this study are summarized in Table 4.. Annual precipitation totals are given in Table 4.2 for 6 stations. They are in reasonable agreement, except that ) both South Branch stations in 2006 and South Branch 2 in 2007 gave much lower totals compared to the other stations, and 2) the Flying Cloud airport gave a high total reading (37.2 in) in Correlation coefficients (R 2 ) between stations were calculated for daily rainfall totals in April through November for 2005, 2006, and 2007 (Table 2.8, Figure 4.). The Rosemount and Empire stations were well correlated in both 2005 and 2006 (R 2 >0.7). Correlations to the Farmington station varied from year to year, with relatively good correlation in 2006 and 2007 and relatively poor correlation in Correlations to the two South Branch stations also varied from year to year, with the best correlations in Correlations to the Flying Cloud airport were relatively low in all years. Precipitation data from the Empire WWTP may be the best single data set, since the station is centrally located in the watershed; data from the Rosemount station may be particularly useful for North Creek. Reliable precipitation data for the watershed at and west of Lakeville are still not available. Table 4.. Precipitation measurements obtained for this study Station Recording Interval Data Record Obtained Empire WWTP (EWWTP) 5 minute Farmington (FRM) day Flying Cloud Airport (FCA) hour Rosemount Station (RSM) 30 minute South Branch (SB and SB2) 4 minute Table 4.2. Annual precipitation (in inches) for 6 stations. Values are for January through December, except where noted. Station Empire WWTP Farmington Flying Cloud Airport Rosemount Station South Branch 5.9 (Jul-Dec) (Jan-Nov) South Branch 2 4. (Jul-Dec) (Jan-Nov) 25

26 Table 4.3. Correlation coefficients (R 2 ) between stations for daily precipitation in 2005, 2006, and R 2 Coefficients, April October 2005 EWWTP SB SB2 FRM RSM FC EWWTP SB SB FRM RSM FC R 2 Coefficients, April October 2006 EWWTP SB SB2 FRM RSM FC EWWTP SB SB FRM RSM FC R 2 Coefficients, April October 2007 EWWTP SB SB2 FRM RSM FC EWWTP SB SB FRM RSM FC Daily Precip,Rosemount (in) y = 0.877x R 2 = Daily Precip, EWWTP (in) Figure 4.. Daily precipitation at Rosemount vs. Empire WWTP for April through October,

27 5. Conclusions As part of an effort to characterize the response of the Vermillion River to surface runoff, the flow records from seven gaging stations were analyzed to determine the data quality, typical low flow values, the fraction of river flow that is base flow (groundwater flow), and the contribution of major tributaries to the total flow. The longest (33 years) and most reliable stream flow record for the Vermillion River has been collected at the USGS stream gaging station near Empire, Minnesota. For the study of stream temperatures in the Vermillion River, the monthly median or the 7Q2 low flows are considered as representative flow conditions. By comparison the 7Q0 low flow represents a relatively extreme low flow. Monthly mean flows are larger than median flows because they include large peak flows, and are therefore considered as an upper bound. For thermal impact studies, a July/August median or 7Q2 flow may be a suitable initial flow condition. The July/August composite values of the mean, median, 7Q2, and 7Q0 flows at the USGS station were 77.9, 48.0, 34.2, and 9.0 cfs, respectively, from Until 2007 the Empire WWTP discharged its effluents into the Vermillion River upstream of the USGS stream gauging station, and supplied about 2 3 cfs most recently. Future low flows in the Vermillion River at and downstream of the USGS station will change because the discharge from the Empire WWTP was permanently diverted to the Mississippi River in To make the adjustment, WWTP discharges were subtracted from the recorded flows at the USGS stream gaging station for the period , and the flow analysis was repeated for the reduced (adjusted) data set. The July/August composite values of the mean, median, 7Q2, and 7Q0 flows at the USGS station obtained for the adjusted data set dropped to 68.0, 43.4, 28.6, and 2.5 cfs, respectively. To estimate the base flow fraction in the Vermillion River, the original USGS flow data were processed using the USGS PART program. When the program was applied to the entire record, , the overall, base flow was estimated to be 84% of the total streamflow. The base flow fraction varied from 72% in March to 98% in January. Analysis of the adjusted USGS data (USGS record -WWTP effluent) yielded an overall base flow fraction of 80% (Table 2.3). The effort by the MNDNR to recalibrate other Vermillion River flow gaging stations appears to have resulted in self-consistent data for 2007 flows in the main stem and tributary stations from SC804 to VR803 (Figure 3.). Although many of the flow gaging stations shown in Figure 3. and examined in this study have flow records of up to eight years, flow data prior to 2007 should be used with caution because of the following findings: The SC804 station appears to give consistent flow readings over the entire eight years of record, although very low mid-summer readings may be uncertain. The VR807 station gives higher than expected reading in 2004 and 2005 The VR803 station has several major periods with no readings, and the peak flows appear to be inconsistent with other stations prior to The SB802 station appears to give reasonable readings in , but gave significantly higher readings prior to

28 Although MC80 and NC808 give fairly consistent flow readings at annual and monthly time scales, only data in 2007 give the expected behavior at daily time scales, i.e. MC80 flows higher than NC808 flows. The relationships of monthly flows at several gaging sites to the flow at the USGS station were examined using polynomial fits of monthly-average flows (Figures 3.2a 3.2e). The analysis implies: The main stem upstream of South Creek (SC804) supplies about 3% of the flow at the USGS station, although the fraction may increase at higher flows. For the years 200, 2006 and 2007 combined, South Creek supplied about 5% of the flow at the USGS station. The combination of North Creek and Middle Creek supplied about 24% of the flow at the USGS station. For , the flow in South Branch was about 4% of the flow at the USGS station, although South Branch enters the Vermillion downstream of the USGS station. South Creek, North Creek, and South Branch contribute significant flows to the Vermillion River main stem, but the fraction of flow attributable to each tributary varies from year to year. Probable causes of the year to year variation are ) differences in the spatial patterns of rainfall from year to year and 2) issues with the gaging sites. As part of the flow study, precipitation data for the Vermillion River watershed were also examined. Both the University of Minnesota Rosemount Experiment Station and the Empire WWTP appear to have reliable, sub-hourly rainfall data for the watershed. Daily rainfall totals for the two stations were fairly well correlated (R 2 = ). Acknowledgments This study was conducted with support from the Minnesota Pollution Control Agency, St. Paul, Minnesota, with Bruce Wilson as the project officer. Streamflow data were supplied by Travis Bistodeau of the Dakota County SWCD and the USGS. Precipitation data were obtained from Karen Jensen at the Met Council, Bill Olsen at Dakota County, and the University of Minnesota Rosemount Research and Outreach Center. We would also like to acknowledge Brian Nerbonne of the MNDNR for his helpful feedback on defining low flow conditions. The authors are grateful to these individuals and organizations for their cooperation. References Erickson, T.O. and H.G.Stefan (2008). Baseflow Analysis for the Upper Vermillion River, Dakota County, Minnesota. Project Report No. 507, St. Anthony Falls Laboratory, University of Minnesota, 69 pp. Martin, J.L. and McCutcheon, S.C Hydrodynamics and transport for water quality modeling. Lewis Publishers, Boca Raton. 28