US Army Corps. Detroit District. Forest Restoration

Transcription

1 R d i S di t D li Th h Reducing Sediment Delivery Through Forest Restoration

2 Drainage Area = 88 mi 2 Mostly forested Close to Duluth Only naturalized wild steelhead population in MN Only MN trib to Great Lakes w/o fish migration barrier

3 Background Federal Harbor is a significant source of sediment to the lake and harbor and has use impairments due to sediment. As such, a TMDL has been established.

4 Source of Sediment Mass Wasting

5 The Problem The watershed was historically composed of old-growth red and white pines with some hardwoods. Modern logging activities have replaced those species with a mono-culture of aspen. Local scientists believe that this shift is species has altered the hydrologic response of the watershed resulting in increased flashiness and increased erosion. Local stakeholders are interested t in initiating iti a forest restoration ti program but would like the following questions answered first: 1. How large of a reduction in sediment delivery is expected if the forest is restored to pre-european settlement species? 2. Are some sub-watersheds more sensitive to restoration than others? 3. What role does the beaver play in storing sediment and reducing flashiness?

6 Summary Watershed response to forest restoration modeled with HSPF Impact of beaver dams on sediment supplies modeled with RAS Hydrologic sensitivity index developed Historic flows statistically analyzed

7 First Date with Temperature and Precipitation it ti

8 Soils and Topography Clay-Rich Till Average Slope = 0.8 %

9 Land Cover and Forest Canopy

10 Watershed Model - HSPF Tree Age Effect 0 years Hydrologic response following clearing and subsequent planting of a pine forest at different recovery ages 2 years 5 years 10 years 20 years 50 years 100 years Forest Restoration Re-establishment establishment of mature coniferous forest representative of pre-european settlement 0% 25% 50% 75% 100%

11 Watershed Model - HSPF Calibrated to Flow at d/s gage

12 Watershed Model Extent of Forest Restoration

13 Watershed Model Extent of Forest Restoration

14 Watershed Model Tree Age

15 Watershed Model Tree Age

16 Beaver Influence on Sediment Castor canadensis is the largest rodent in North America, growing to 60 lbs over an average life span of 24 years. Beaver dams can significantly affect stream flashiness Beaver dams can store a large quantity of sediment A large pulse of sediment can be mobilized upon breaching

17 Beaver Confusion Castor canadensis Intern canadensis

18 Beaver Influence on Sediment

19 Summary of flow at USGS Gage

20 Flashiness Index No Trend

21 Flashiness Index

22 Flashiness Index

23 Hydrologic Sensitivity Index Ranks hydrologic sensitivity of sub-basins based on slope, distance to outlet and percent disturbed land

24 Conclusions Forest restoration to Pre-European species will have a minimal effect on altering stream flows except in a few sub-watersheds Beaver dams will store water and reduce the flashiness of the stream at low flows, but have no effect once the 2-yr flow is exceeded Beaver dams only temporarily impound sediment. Larger waves of sediment are likely released when a dam or multiple dams fail

25 Questions? Contact: Dr. Jim Selegean, P.E., P.H. U.S. Army Corps, Great Lakes Hydraulics and Hydrology Office 477 Michigan Ave Detroit, MI army mil

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27 Figure 39: Impacts of different beaver dam densities with unsteady flow modeling. a) far downstream cross- section b) downstream cross-setion setion c) cross-section immedietly downstream of dams d) within beaver dam section e) upstream of beaver dams - The black dashed line indicates time of first dam breach

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