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Most studies of organic matter transport have been conducted on small headwater streams and have found that floods strongly influence downstream transport. We studied five sites on four rivers, each with different hydrographs and flood frequencies, in the Adirondack Mountains, New York, USA, during the summer and autumn of 2005. Sites on three rivers had more natural hydrographs, whereas recreational releases in the Indian River increased discharge 10-fold and to a lesser extent in the upper Hudson River below their confluence. Fine and coarse particulate organic matter (FPOM and CPOM, respectively) transport decreased from June to August at base flow in most rivers, concurrent with summer decreases in discharge; however, during releases CPOM transport increased in the Indian River and at the downstream Hudson River site below the confluence with the Indian River. Also, in the summer there was a shift from terrestrial POM in transport in June to more filamentous algae in July and August because of sloughing of periphyton. In October, POM transport increased at most sites from August levels because of autumnal leaf inputs. In summer 2006, we sampled all sites during and after a natural flood, and both FPOM transport and CPOM transport were highest in rivers with more natural hydrographs (>10,000% increase from base-flow transport levels after the flood) than in the Indian and downstream Hudson sites ($1000% increase or less). Our results suggest that recreational releases do reduce POM sources from floodplain and benthic habitats, based on lower POM transport during a natural flood.

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