FIELD ASSESSMENT OF EROSION POTENTIAL AND SEDIMENT LOADING OF THE GRASSE RIVER, ST. LAWRENCE COUNTY, NEW YORK

The Grasse River, in the St. Lawrence watershed, is being evaluated for soil erosion potential and sediment loading. This project is related in part to the United States Army Corps of Engineering’s (USACE) Great Lakes Tributary Modeling (GLTM) 516e Program that is assessing watersheds for erosion, sedimentation, and pollution issues. Students are using the USACE’s GLTM 516e Sediment Transport Analysis and Reginal Training (START) Field Reconnaissance assessment approach and rating system for measuring erosion potential and sediment loading to determine overall quality of the watershed. Upstream and downstream, both left and right banks are evaluated at each sample site. Five categories (streambank, streambed, streamflow, vegetation, flow obstructions) are evaluated by numerical rating subcategories of related factors (e.g. streambank erosion, slope, material, stability, etc.). Each subcategory receives a numerical score (low score= little/ no erosion, high score= high erosion). Subcategory scores are totaled and the main category is ranked, the site is then given an overall rating from Normal, Minor, Moderate, to High erosion potential. Stream sediment measurements are taken using the Ohio Sediment Stick (OSS) per START procedure, and with a Turbidimeter for comparison. Measurements are interpreted into a Total Suspended Solids (TSS) value. Preliminary site evaluations indicated that there are 9 Normal and 5 Minor sites based off the START general rating. Most sites had streambeds and banks made of sand, gravel, and cobbles, little/no vegetation in streambeds, well vegetated banks, variable indicators of erosion and deposits, wide channels, shallow-moderate depth, low-moderate flow, little/no lateral migration, and variable obstructions (bedrock outcrops, bridge piers, and dams). The OSS results translate to a TSS <6.2 mg/L for all sites. TSS of <10.0 mg/L is considered excellent water quality. Turbidity readings were all comparable, ranging from 0.93 to 2.45 NTU. The results indicate clear water. Initial comparison indicates that there is no statistically significant difference in water quality between the Normal and Minor rated sites (p-values <0.05, for one-way ANOVA tests, a=0.05). Based on current data it can be determined that the overall water quality of the Grasse River is good to excellent.