PARTICULATE ORGANIC-MATTER TRANSPORT IN THE LOWER COLORADO RIVER, SOUTH-WESTERN USA

The lower Colorado River from Laughlin, Nevada, to the border with Mex ico is one of the most regulated rivers in North America. An analysis of particulate organic matter (POM) transport through the system was m ade to determine POM concentrations, composition and point of origin, as well as to determine the effect darns had on this transport. Partic ulate organic matter concentrations generally increased from upstream to downstream, and were dominated by very fine organic detritus (< 25 mu m size fraction). Reservoirs released more POM than they received d uring spring and summer, but less during autumn and winter. Reservoirs recycled POM, taking in organic detritus and discharging small limnop lankton. Backwater complexes functioned as filters during storm events , trapping coarse material and releasing fine material. Agricultural d rains made only minor contributions of POM, but provided important nut rients for in-channel production of POM. Water diversions forced POM o ff-channel, whereas sluicing operations reduced inorganic loads in the canals. Despite the presence of six mainstem dams, the lower reaches support a diverse assemblage of detritivorous fishes, similar to that found in the lowermost reaches of natural, unregulated rivers. The foo d base for this fishery stems from autochthonous production of POM.