Sediment and nutrient dynamics following a low-head dam removal at Murphy Creek, California

Ahearn, Dylan S., Randy A. Dahlgren

Limnol. Oceanogr., 50(6), 2005, 1752-1762 | DOI: 10.4319/lo.2005.50.6.1752

ABSTRACT: We studied the impact of the removal of a 3-m dam on sediment and nutrient export. In the year after dam removal, sediment and nitrogen (N) export increased by an order of magnitude over the previous 2-yr mean. Longitudinal surface water samples were collected, sediments were cored, and the channel was surveyed during different seasons to determine the mechanisms driving sediment and nutrient dynamics in the recovering system. The majority of sediment transport occurred in pools and in the lowest 50 m of the 620-m restored reach. Phosphate export occurred primarily during large storms, with the restored reach acting as a phosphate sink during most flow conditions. The majority of surface water N originated from areas within the sediment wedge that had high extractable N concentrations (average NH4-N = 50 µg g-1 and NO3-N = 38 µg g-1) and dried out on a seasonal basis. Near the former dam site, year-round water saturation apparently inhibited nitrification and export of N as nitrate. This wetland area was the only portion of the restored reach that was an ammonium sink. After dam removal, N leaching from sediments occurred in autumn 2003 and again during the subsequent autumn, suggesting that N leaching from reservoir sediments is largely a seasonal process that may affect downstream aquatic ecosystems for a number of years.

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