TRANSPORT DYNAMICS OF FINE PARTICULATE ORGANIC-MATTER IN 2 IDAHO STREAMS

Natural suspended fine particulate organic matter (FPOM, 53-102-mum diam) was labeled with C-14 and reinjected to estimate transport distances in the water column and retention times within the sediments of two Idaho streams. Transport of labeled FPOM particles declined exponentially with downstream distance, yielding mean transport distances of 800 and 580 m in Smiley Creek in 1989 and 1990 at a mean water velocity of 0.27 m s-1 and mean depth of 0.34 m in both years, and a distance of 630 m in the upper Salmon River at a mean velocity of 0.29 m s-1 and a depth of 0. 14 m. These travel distances are equivalent to vertical deposition velocities of 0.7-1.6 mm s-1, or approximately 7-12% of the temperature-corrected quiescent-water fall velocities of the FPOM particles. The estimated deposition flux of approximately 1.5 g (AFDM) m-2 d-1 would turn oyer the standing stock of 0. 8 g M-2 in surficial sediments twice daily. Sampling of benthic FPOM 24 h after release indicated that approximately 99% of the C-14-labeled FPOM initially deposited in the 1,005-m study reach had been resuspended and exported; subsequent clearing was much slower. An advection-dispersion model satisfactorily simulated the deposition of C-14-labeled particles nd their subsequent resuspension and export from the reach. Model-estimated retention times were 1.5-3 h for 99% of deposited sediments and 17 d for the remaining 1%. Thus, particles in surficial sediments exchange rapidly with the water column and migrate downstream several kilometers per day in alternating deposition and resuspension events. These results support the overall concept that conditions throughout a river system are strongly connected longitudinally and that OM introduced in headwater reaches can be transported large distances for later use or storage elsewhere in the river or for eventual export to estuaries.