No abstract

1. Chlorophyll a (Chl a) distribution across a 0.36 km 2 restored floodplain (Cosumnes River, California) was analysed throughout the winter and spring flood season from January to June 2005. In addition, high temporal-resolution Chl a measurements were made in situ with field fluorometers in the floodplain and adjacent channel. 2. The primary objectives were to characterise suspended algal biomass distribution across the floodplain at various degrees of connection with the channel and to correlate Chl a concentration and distribution with physical and chemical gradients across the floodplain. 3. Our analysis indicates that periodic connection and disconnection of the floodplain with the channel is vital to the functioning of the floodplain as a source of concentrated suspended algal biomass for downstream aquatic ecosystems. 4. Peak Chl a levels on the floodplain occurred during disconnection, reaching levels as high as 25 lg L )1 . Chl a distribution across the floodplain was controlled by residence time and local physical/biological conditions, the latter of which were primarily a function of water depth. 5. During connection, the primary pond on the floodplain exhibited low Chl a (mean ¼ 3.4 lg L )1 ) and the shallow littoral zones had elevated concentrations (mean ¼ 4.6 lg L )1 ); during disconnection, shallow zone Chl a increased (mean ¼ 12.4 lg L )1 ), but the pond experienced the greatest algal growth (mean ¼ 14.7 lg L )1 ). 6. Storm-induced floodwaters entering the floodplain not only displaced antecedent floodplain waters, but also redistributed floodplain resources, creating complex mixing dynamics between parcels of water with distinct chemistries. Incomplete replacement of antecedent floodplain waters led to localised hypoxia in non-flushed areas. 7. The degree of complexity revealed in this analysis makes clear the need for highresolution spatial and temporal studies such as this to begin to understand the functioning of dynamic and heterogeneous floodplain ecosystems.

Temporal patterns of stream water chemistry were analyzed across the Cosumnes River Watershed (1989 km 2 ) for water years 1999-2002 to quantify hydrobiogeochemical dynamics in the last free-flowing watershed draining the western Sierra Nevada, California. The Mediterranean climate of California produces a distinct annual hydrologic pattern with three seasons: baseflow, stormflow, and meltflow. The baseflow season (July -October) is dominated by groundwater chemistry that primarily originates from high elevations, and thus does not vary much across the basin. During the baseflow season discharge is negatively correlated to ionic concentration, and sediment and nutrients are generally below detection levels. The stormflow season (November -March) is separated into a flushing period (where discharge is positively correlated to river water conductivity) and a dilution period (where discharge is negatively correlated to conductivity). During average flow years, virtually the entire annual load of nutrients and sediment moves through the watershed during the stormflow season. Because stormflow hydrologically links the land with local waterways, the stormflow season shows the greatest variance among sites across the diverse landscape of the Cosumnes Watershed. Chemistry of the meltflow season (April -June) is dominated by dilute upland snowmelt, and there is little chemical variation across the watershed. Storm-scale analysis in water year 2002 revealed that progressive flushing occurs with each storm event and that source area dynamics play an important role in chemograph response. With 19 of the 20 major rivers in the western Sierra Nevada having dams, these data provide scientists and regulators with a valuable reference to address how impoundment affects water quality. q

This study examines the effects of flow regulation on water quantity and quality by comparing an impounded system (Mokelumne River) with an adjacent unimpounded system (Cosumnes River). Between 1999 and 2002, the Cosumnes River displayed a strong seasonal cycle for each constituent analysed (total suspended solids, NO 3 -N, total nitrogen, PO 4 -P, total phosphorus, dissolved silicon, specific conductivity, flow), while reservoirs in the lower Mokelumne buffered and attenuated physical and chemical fluctuations creating a weak seasonal pattern. Dissolved silicon and total suspended solids were the two constituents most efficiently sequestered by the reservoirs. While the reservoirs acted as traps for most constituents, NO 3 -N and PO 4 -P were produced during the drier years of the study, 2001 and 2002. In contrast, the unimpounded reference reach in the Cosumnes was an annual source for all constituents measured. The Cosumnes delivers its highest NO 3 -N concentrations during the winter months (December-April), while peak concentrations in the Mokelumne occur during the snowmelt (May-July) and baseflow (August-November) seasons. Due to downstream N-limitation, this temporal shift in NO 3 -N export may be contributing to accelerated algal growth in the reach immediately downstream and eventually to algal biomass loading to the downstream Sacramento-San Joaquin Delta.

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 NH 4 -N ϭ 50 g g Ϫ1 and NO 3 -N ϭ 38 g g Ϫ1 ) 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.