|SIMON, ANDREW - RETIRED ARS EMPLOYEE|
|KLIMETZ, LAUREN - FORMER ARS EMPLOYEE|
|BANKHEAD, NATASHA - FORMER ARS EMPLOYEE|
|Ursic, Michael - Mick|
Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 12/1/2012
Publication Date: 12/1/2012
Citation: Langendoen, E.J., Simon, A., Klimetz, L.E., Bankhead, N., Ursic, M.E. 2012. Quantifying sediment loadings from streambank erosion in selected agricultural watersheds draining to Lake Champlain. USDA-ARS-NSL Research Report No. 79. pp. 1-78.
Interpretive Summary: At its mouth on Lake Champlain the Missisquoi River has a history of exceedance of phosphorus concentration target levels endorsed by the governments of Vermont, Québec, and New York. As portions of the Missisquoi exhibit streambank erosion, streambanks could be a significant source of the phosphorus to Lake Champlain. Scientists at the U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory used the Bank Stability and Toe Erosion Model (BSTEM) to determined current streambank erosion rates and loadings to Lake Champlain and evaluated the percent reduction that could be received by three different mitigation scenarios. The mitigation scenarios consisted of stabilizing the streambanks by: (1) establishment of mature riparian vegetation on the bank top, (2) grading the banks at a 2:1 (horizontal:vertical) slope, and (3) combining the grading of the banks (scenario 2) with a vegetative treatment of both the bank face and top. It was found that streambank erosion contributes approximately 36% of the total suspended sediment (TSS) load (31,600 metric tons/yr) and 36% of the total phosphorus (TP) load (52.0 metric tons/yr) entering Lake Champlain. Mitigation scenarios (1) and (2) provided similar percent reductions of suspended sediment load and total phosphorus load to Lake Champlain: 21% TSS and 29% TP for scenario (1) and 12% TSS and 14% TP for scenario (2). The calculated reductions in eroded bank material volumes were greatest for scenario (3). TSS loadings were reduced by 85% to 4,870 metric tons/yr, whereas TP loadings were reduced by 84% to 8.4 metric tons/yr. These findings will be used by the Lake Champlain Basin Program, the State of Vermont Department of Natural Resources, and the U.S. Environmental Protection Agency to develop a phosphorus Total Maximum Daily Load (TMDL) for the Missisquoi River watershed.
Technical Abstract: At its mouth on Lake Champlain the Missisquoi River has a history of exceedance of phosphorus concentration target levels endorsed by the governments of Vermont, Québec, and New York. Observations along the study reach of the Missisquoi River and several of its tributaries have indicated that the river’s streambanks could be a significant source of the suspended sediment and hence phosphorus to Missisquoi Bay. Indeed, significant portions of the study reach were estimated to have greater than 50% of their banks failing. The main objective of this study was to determine rates and loadings of sediment from streambank erosion along main stem reaches of the Missisquoi River located in the United States (US) and select tributaries. Further, three mitigation scenarios were analyzed to determine the percent reduction in loadings that can be obtained by stabilizing streambanks: mitigation scenario 1) 25 year old mature trees on the bank top, mitigation scenario 2) banks graded to a 2H:1V slope, and mitigation scenario 3) banks graded to a 2H:1V slope in combination with 5-year old vegetative treatment on bank top and face. Bank stability and toe erosion analyses were carried out using the model BSTEM at 27 study sites along the study reach using a 30-year (1980-2010), historic flow record. Predicted volumes of sediment eroded from the streambanks at each site ranged from 0.0559 to 1780 cubic meters of sediment per one kilometer reach per year (cu. m/km/yr) under existing conditions with a median value of 69.3 cu. m/km/yr. Contributions of sediment from streambank erosion along the US study reaches of the Missisquoi River were found to be about 36% (31,600 t/yr) of the total suspended-sediment load entering Missisquoi Bay. Maximum associated phosphorus loadings of up to 1,540 kg/km/yr were calculated in the lower portion of Tyler Branch and the confluence of the Missisquoi and Trout Rivers. The median calculated phosphorus loading was 41.7 kg/km/yr. The calculated streambank erosion volumes contributed about 36% (52.0 t/yr) to the total phosphorus load entering Missisquoi Bay. Mitigation scenarios 1 and 2 provided similar percent reductions in sediment loadings. Median erosion volumes were reduced by 60% to 27.7 cu. m/km/yr for scenario 1 and 50% to 34.8 cu. m/km/yr for scenario 2. The calculated reductions in eroded bank material volumes were greatest for scenario 3. The median value was reduced by 100% (0 cu. m/km/yr), and the maximum value was reduced by 59% (740 cu. m/km/yr). Contributions of sediment from streambank erosion along the US study reaches of the Missisquoi River to the total suspended-sediment load entering Missisquoi Bay were reduced by: scenario 1) 21% (25,000 t/yr), scenario 2) 12% (27,700 t/yr), and scenario 3) 85% (4,870 t/yr). The mitigation scenarios have a similar impact on phosphorus loadings as they have on sediment loadings, since phosphorus loadings are directly related to bank material loadings. The calculated contribution to the total phosphorus load into Missisquoi Bay is reduced by 29% (37.1 t/yr) for scenario 1, 14% (44.8 t/yr) for scenario 2, and 84% (8.43 t/yr) for scenario 3.