Suspended Sediment, Carbon and Nitrogen Fluxes during the Dewatering of a Low-Head Impoundment and a Subsequent Flood Event

Previous to dam removal, for restoration purposes, a low-head impoundment was dewatered (28 April 2005) during baseflow conditions using existing flood gates. Approximately one week later (6 May 2005) a near bankfull event passed through the recently dewatered system. Inputs upstream of the impoundment accounted for 657 kg of TSS during the 30 hours of dewatering. The impoundment was a point source of TSS contributing and additional 1.08 tonnes to the system at a rate of 35.9 kg hr-1. Reach-level fluxes of dissolved organic carbon (DOC) and dissolved N were also amended by the impoundment with 422 kg and 41 kg respectively. Upstream sources supplied 1094 kg of DOC and 105 kg of N to the impounded section of the reach. These impoundment contributions were supplied at rates of 14.08 kg hr-1 for DOC and 1.39 kg hr-1 for N. In contrast, during the 6 day flood event the impoundment acted as a sink for TSS by storing 31.4 tonnes. During this same event the impoundment contributed 6.4 tonnes of DOC and 0.70 tonnes of N to downstream environments at rates of 48.96 kg hr-1 and 5.39 kg hr-1 respectively. Upstream loading provided 38.2 tonnes of DOC and 2.8 tonnes of N. These data suggest that dissolved C and N are not subjected to the same transport limitations as TSS. Further, the dewatering of this particular dam (during baseflow conditions) impacted downstream water chemistry less than the subsequent flood event in both mass and rate of loading. These data support the concept that post-dewatering/removal flood events have the potential to act as a more significant disturbance than the actual removal/dewatering if the flood occurs within the `window of sensitivity' during the initial phases of system recovery.