GRASSE RIVER WATERSHED EROSION POTENTIAL, SEDIMENT LOADING, AND NUTRIENT LOADING ANALYSIS, ST. LAWRENCE COUNTY, NEW YORK

The Grasse River was evaluated for soil erosion potential and sediment and nutrient loading to determine impacts of agricultural land use. Students used the United States Army Corps of Engineering’s (USACE) Great Lakes Tributary Modeling (GLTM) 516e Sediment Transport Analysis and Regional Training (START) Field Reconnaissance rating system to evaluate erosion and loading potential. Five main categories (streambank, streambed, streamflow, vegetation, flow obstructions) were evaluated. Overall site rankings indicated little/no erosion and loading potential (9 Normal and 5 Minor rankings), largely due to coarse grained and bedrock channels with well vegetated riparian zones. Water quality was tested for actual sediment and nutrient loading. Sediment loading was present (averages: conductivity = 123.18 μS, total dissolved solids (TDS) = 90.47 ppm, total suspended solids (TSS) < 6.2 mg/L, turbidity = 2.07 NTU). Nutrient loading was present. Fall and spring median combined nitrate-nitrite (3.05 and 2.8 mg/L-N) and total phosphorus (0.335 and 0.275 mg/L PO₄^3-) were greater than national median values for agriculturally impacted streams. Orthophosphate (0.06 and 0.07 mg/L PO₄^3-) was below the national median. Only total phosphorus was greater than regulatory goals and standards. Turbidity and all nutrient concentrations fluctuated progressively downstream. Basic water quality parameters, all solids related parameters, and nitrite were statistically different (p-values < 0.05 for ANOVA tests, a = 0.05) between fall and spring sampling. Other nutrients’ season variation was obscured in statistical analysis by high standard deviations. Higher discharges did not always equate to higher concentrations. Variations could be due to tributaries increasing loading or diluting, runoff causing erosion, tile drains, variable tilling/irrigation, variable land use, and/or obstructions causing settling. There was no statistical difference for nutrient and sediment concentrations based on Normal and Minor site rank. Despite the presence of sediment and nutrients, there is low TSS, healthy pH (7.85) and dissolved oxygen (11.76 mg/L) levels, presence of aquatic life, and lack of algal blooms. Overall, while the Grasse River is impacted by agricultural land use it maintains good water quality.