Stream bank soil and phosphorus losses within grazed pasture stream reaches in the Rathbun Watershed in southern Iowa

Abstract

Livestock grazing of riparian zones can have a major impact on stream banks if improperly managed. The goals of this study were to determine the sediment and phosphorus losses from stream bank soils under varying cattle stocking rates and identify other factors that impact stream bank erosion in the Southern Iowa Drift Plain. The study was conducted on thirteen cooperating beef cow-calf farms within the Rathbun Lake watershed in South Central Iowa. Stream bank erosion rates over three years were estimated by using the erosion pin method. Eroded stream bank lengths and area, soil bulk density and stream bank soil-P concentrations were measured to calculate soil and total soil-P lost via stream bank erosion. Results revealed that the length of severely eroded stream banks and compaction of the riparian area were positively related to an increase in number of livestock grazing on the pasture stream reaches. While there was no direct relationship between bank erosion and stocking rate, the erosion rates from two sites enrolled within the Conservation Reserve Program (CRP) were significantly lower than those from all grazed pasture sites especially when seasonal effect, specifically winter/spring, was considered. This result suggests that use of riparian areas as pasture has major negative impacts on water quality and channel integrity through increased sediment and phosphorus from bank erosion, and that impact could be reduced through management of livestock grazing within these riparian areas.