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.