Grazing Relations — Abstract of Paper


Sediment Dynamics and Sources in a Grazed Hardwood Rangeland Watershed
Melvin George,¹ Neil McDougald,² Kenneth Tate,¹ Royce Larson³


From 1994 to 1998 we documented sediment transport dynamics and sources in a 137 ha grazed hardwood rangeland watershed on granitic soils at the San Joaquin Experimental Range in Madera County. Sediment transport for this watershed was determined by measuring total suspended solids, bedload and flow at an H-flume installed in 1994. Sediment movement as bedload is the primary means of sediment transport in this watershed, with minimal transport of suspended solids. This is attributed to the large sediment particle size and low stream power characteristic of this low gradient intermittent watershed. Bedload transport can be predicted by stream flow (p<0.0001, R²=0.68). Upland sediment sources were surveyed using paired hillslope plots to estimate upland erosion for three slope classes. Sediment traps were installed to compare erosion on cattle trails and adjacent non-trailed areas. Ten stream cross section profiles were averaged to determine grazing treatment and year effects on in-stream erosion and deposition along three intermittent stream channels. Due to rapid infiltration, runoff and sediment yield from the hillslope plots averaged only 151 mm and 36 kg/ha, respectively. Unvegetated, disturbed soil surfaces in cattle trails were a significantly greater (p<0.002) source of sediment (0=238 g/trap, n=8 traps) than adjacent well vegetated soil surfaces (0=6 g/trap, n=8 traps). From 1994 to 1998 stream channel morphological parameters did not change in response to no grazing, winter moderate or concentrated grazing, or dry season moderate or concentrated grazing. There was a year effect on channel depth due to the dynamics of bedload transport in response to variation in peak storm events from year to year. The results of these studies suggest that sediment movement from cattle trails into stream channels is the main grazing induced sediment source in this watershed.