Water quality response to a pulsed-flow event on the Mokelumne River, California

Abstract

Controlled water releases from reservoirs (i.e. artificial floods) are used as a management technique to remove tine sediments and detrital materials from spawning gravels, mobilize gravel bars and clear encroaching brush from stream banks. The effects of a managed release event on water quality were investigated on the lower Mokelumne River in the western Sierra Nevada, California. The managed release was characterized by an increase in flow over a 4-day period (from 11 to 57 m3 s-1). Automatic pump samplers were used to collect samples for water quality from 0.7, 16.4, 37.4 and 54.4 km below Camanche Dam. These sampling sites provided water quality data for three distinct stream reaches: a gravel and sand-textured substrate reach (0.7-16.4 km), a reach characterized by lentic conditions associated with a small reservoir (16.4-37.4 km), and fine sand and silt-textured substrate reach (37.4-54.4 km). Water samples were analysed for total suspended solids (TSS), total nitrogen, ammonium (NH4-N), nitrate (NO3-N), total phosphorus, soluble reactive phosphorus (SRP), dissolved organic carbon (DOC), foecal coliforms and E. coli. Chemographs for all constituents exhibited spikes in concentration with each increase in streamflow for the rising limb. Fluxes of TSS, total P and total N released from the 0.7 to 16.4 km reach were 322, 0.32 and 0.70Mg, respectively. The small reservoir acted as a sink for particulate materiais retaining about 50% of TSS, 48% of total P and 43% of total N. However, the reservoir acted as a source of dissolved nutrients (NO3-N = 0.28 Mg and SRP = 0.055 Mg). The stream reach below the reservoir (37.4 to 54.4 km) was a source of particulate materials, dissolved nutrients and bacteria, possibly due to agricultural and urban inputs. Copyright © 2007 John Wiley & Sons, Ltd.