Sediment Delivery to the Watonwan River

What Are Total Suspended Solids (TSS) Solid material that is suspended in the water column –Clay and Silt –Organic matter

Monitoring Season Total Suspended Solids (TSS) Flow-Weighted Mean Concentration

Major Storms and Sediment Loads WMS3 216,000 acres

Watonwan River Site WMS3 Runoff

WMS Daily Average Flow and Rainfall 4 Storms accounted for 78% of the sediment load

2000 WMS3 Maximum 1 Hour Rainfall Intensities

2000 Summary 17 rain events over 0.5” 4 events accounted for most of the flow and sediment All had rainfall intensities greater than 0.5”/hr Timing is important - After the crop canopy is closed, runoff and erosion from tillable acres appears to be substantially reduced

WMS Daily Average Flow and Rainfall 3 Storms accounted for 71% of the TSS Load

WMS Daily Average Flow and Maximum 1 hr. Rainfall Intensities

Early through Mid-Spring Pre-Planting Period

Mid-Spring Through Mid- Summer Period Post-Planting Pre-Canopy

Mid-Summer Through Fall Post-Canopy Period

2002 Summary Early Spring – little overland flow Low intensity storms/residue Early through midsummer – 2 storms alone accounted for 58% of the sediment load but only 25% of the flow High Intensity storms, minimal residue, altered soil structure Late summer – canopy closed erosion and runoff reduced

Watonwan 2001 – Flood Year

WMS Daily Average Flow and Rainfall

WMS Daily Average Flow and Rainfall – Flood Flows Excluded

Watonwan River TSS Flow Weighted Mean Concentrations

Potential Sediment Sources Overland flowRunoff from construction sites Storm water runoff Stream bank erosion

2000 Watonwan Storms

Open Tile Intakes

Result

What Can Be Done to Reduce The Amount of Sediment Delivered to the Watonwan River? Best Management Practices –Urban – Stormwater and Construction BMP’s –Rural – agricultural BMP’s Continue existing BMP’s: residue management; conservation tillage; environmentally sensitive land retirements Increased: buffers; wetland restorations (water storage); grass waterways; alternate tile intakes

Agricultural BMP’s

Conclusions High intensity storms of large aerial coverage occurring during the post-planting pre-canopy period delivered the majority of the total suspended solids loads to Watonwan River site WMS3 during 2000 and 2002 The TSS load delivered with the flood flows of 2001 was a significant proportion of the seasonal sediment load. However, the TSS flow weighted mean concentration was the lowest of the four monitoring seasons

Conclusions cont. It appears current BMP’s succeed in reducing sediment delivery to the Watonwan River during the pre-planting period. Runoff and sediment delivery during the closed-canopy period is less than the during the post-planting/pre- canopy period Additional BMP’s are needed to address sediment loads and concentrations delivered to the river during the post-planting/pre-canopy period –Wetland restorations –Buffer strips –Grass waterways –Alternatives to open tile intakes

Mid-Spring Through Mid- Summer Period Post-Planting Pre-Canopy