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

Natural suspended fine particulate organic matter (FPOM, 53-102-mum di am) was labeled with C-14 and reinjected to estimate transport distanc es in the water column and retention times within the sediments of two Idaho streams. Transport of labeled FPOM particles declined exponenti ally with downstream distance, yielding mean transport distances of 80 0 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 o f 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 th e temperature-corrected quiescent-water fall velocities of the FPOM pa rticles. 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 s ediments twice daily. Sampling of benthic FPOM 24 h after release indi cated that approximately 99% of the C-14-labeled FPOM initially deposi ted in the 1,005-m study reach had been resuspended and exported; subs equent clearing was much slower. An advection-dispersion model satisfa ctorily simulated the deposition of C-14-labeled particles nd their su bsequent resuspension and export from the reach. Model-estimated reten tion times were 1.5-3 h for 99% of deposited sediments and 17 d for th e remaining 1%. Thus, particles in surficial sediments exchange rapidl y with the water column and migrate downstream several kilometers per day in alternating deposition and resuspension events. These results s upport the overall concept that conditions throughout a river system a re strongly connected longitudinally and that OM introduced in headwat er reaches can be transported large distances for later use or storage elsewhere in the river or for eventual export to estuaries.