Poplar River Turbidity Assessment

The lower Poplar River is listed as impaired due to exceedances of its 10 NTU turbidity standard. Sampling data demonstrate that exceedances occur frequently at flows greater than 68 Cubic Feet per Second (CFS); the 40% highest flow. Turbidity measurements are highly correlated to sediment measurements, indicating that fine sediment fractions are likely the primary cause of turbidity within the lower Poplar River. Analysis of these sources indicated that the upland sources are most likely to occur during precipitation events when there is little vegetative cover and/ or when the ground is saturated. Soil particles are detached from the soil matrix and transported to the river via overland flow. Near stream sources likely occur when flow and stage are high and the stream impinges on the barren valley walls aggravating slumping and/ or mass wasting of existing slumps. Table E2 reports the estimated average, minimum, and maximum loads from each source and its percent contribution to the total load. While combining the upland and near channel sediment estimates may be somewhat of an "apples" to "oranges" comparison based on the different time periods they were derived from (e.g. the modeling averages loading estimates predicted from a 5 year long simulation and the near channel assessment was based on observations and photographs spanning decades) it is the best estimate available, and provides a quantitative comparison of all identified sources.