The tidal Hudson River has at least seventy-nine small to large tributary streams in addition to the Upper Hudson andMohawkRivers, which enter the tidal Hudson at the Troy Dam, the upstream limit of tidal influence. There are also an unknown number of smaller, often ephemeral, streams that contribute water to the tidal Hudson River. These tributaries (other than the Upper Hudson/Mohawk) contribute about 20 percent of the freshwater input to the Hudson River (Cooper, Cantelmo, and Newton, 1988). Various researchers have considered these tributaries as sources of important materials such as carbon, sediments (Howarth et al.,

1991; Howarth, Schneider, and Swaney, 1996; Swaney, Sherman, and Howarth, 1996), and contaminants (Hirschbergetal., 1996).This isareason-able view of the role of tributaries since anything dissolved, suspended, or entrained in tributary flow will end up in the tidal Hudson River. But it is not a complete picture of the role of tributaries in this ecosystem. Our observations over at least the last fifteen years have shown that there is an exchange of fishes between the tidal Hudson River and its tributaries in both directions that occurs over several time scales and that varies from critical to the fishes and ecosystems, to incidental. Our purpose in this chapter is to document what we have observed about this exchange of fishes between tributaries and the tidal Hudson River and to propose some hypotheses about the significance of this phenomenon.

In order to organize our observations, we have categorized the fishes we have seen in tributaries as anadromous, catadromous, potamodromous, resident, and other species. These categories overlap and are not necessarily mutually exclusive. We have chosen to ignore the movements of fishes between the Upper Hudson/Mohawk River and the tidal Hudson River (through the locks on the Troy Dam) not because they aren't significant, but because we think these movements are complex and poorly documented. We would have liked to characterize the tributaries to the tidal Hudson River on some scale of quality, but we visited many of these tributaries only briefly and we have thoroughly sampled only a few. We have also omitted presentation of methodologies in this chapter. Methods are described in detail in the various reports and publications we have cited and we therefore direct anyone interested to those sources.

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