Effects of Small Impoundments on Total Watershed Sediment Yield in Northeast Kansas, April through August 2011

Abstract

The effect of very small impoundments (ponds) on total watershed sediment yield is not widely understood. While it is generally assumed that small ponds trap sediment, little empirical data has been collected to quantify this effect. A multi-agency Sediment Baseline Study (SBS) was commissioned by the Kansas Water Office to characterize the baseline sediment loads and yields in three watersheds in northeast Kansas in order to assess the effectiveness of best management practices and potentially reduce sedimentation of downstream reservoirs. The basins selected for the SBS were the watersheds for Atchison County Lake, Banner Creek Lake, and Centralia Lake. Initial results of the SBS indicated that the number of small impoundments in a watershed may play a strong role in affecting sediment yield. In order to investigate this hypothesis, three small ponds were monitored for flow and sediment data from April to August 2011. Five storm events occurred during this time period. The two impoundments in the Atchison Lake watershed trapped 61 tons of sediment during this study, while a downstream gage recorded a total of 261 tons. The sediment trapped in the impoundments represented 19 percent of the total watershed sediment load recorded at the downstream sediment gage. The Banner Creek Lake watershed impoundment was observed to trap 39 tons of sediment from June 2 to June 23, 2011, but wide variations in water elevation prevented the collection of a complete record over the study period, and calculated loads were subject to large sources of error due to the necessity of estimating elevation-storage below the elevations surveyed. These results show that small impoundments can trap a significant quantity of sediment, potentially reducing sedimentation in downstream reservoirs. A decrease in trapping efficiency was observed over the period of study, which was likely related to decreasing pond volumes as well as suspended sediments remaining in the water column or being re-suspended by other mechanisms between storm events. Based on these results, watershed managers might consider adding, dredging, or repairing small impoundments as a method to reduce downstream reservoir sedimentation, though environmental and economic factors would have to be considered.