Organic seston was studied in a 3rd-4th order impounded river in southwestern Virginia, U.S.A. the hypolimnetic release reservoir had an area of 67-ha and was located in a 4860-ha watershed (90% forested). the objective of this study was to determine reservoir effects on seston quality by measuring microbial activity (respiratory electron transport) and chlorophyllaon river and reservoir seston. Seston was significantly more concentrated upstream of the reservoir than at other sites due to lentic sedimentation. Chlorophyllaassociated with reservoir seston was significantly higher than either imports or exports suggesting that reservoir seston was of higher quality than riverine seston. Seston microbial activity data supported this conclusion to some extent, being higher in the reservoir than at upstream sites and immediately below the dam. Five km downstream however, chlorophyllalevels were still low but seston microbial activity increased. We believe this to be a result of both greater numbers of respiratory microbes and increased respiratory activity of individual microbes. Through sedimentation of refractory riverine seston, in-reservoir autotrophic production, and high microbial activity downstream impoundments may contribute to maximizing lotic ecosystem efficiency.