River damming results in modifications of flow regimes and geomorphic processes, which lead to changes in riparian vegetation downstream. Changes in existing vegetation may result from modifications in the physical habitat and hydrology, and dams may also interfere with critical mechanisms of reproduction and early life stages of riparian species. Dispersal of propagules, and the deposition of seeds in sites appropriate for long-term survival are important determinants of the persistence of many riparian species and influence the composition and structure of riparian plant communities. The objective of this study is: 1.) to determine the relative importance of various dispersal and storage mechanisms (wind, water, and soil seed banks) in colonizing new sites and in structuring riparian plant communities along mountain streams; 2.) to measure the degree to which dams interrupt these processes; 3.) to document where seeds of various riparian species are transported in the water column and the depositional patterns of seeds in relation to depositional landforms and hydrologic regimes. Field studies on three regulated, high gradient mountain streams will: 1.) document the relative contributions of aerial and water dispersed seeds to the colonization of surfaces along river margins and bars, 2.) sample aerially and water dispersed seeds at several cross sections both upstream and downstream from impoundments and 3.) measure the transport mechanics, duration of buoyancy and viability, and patterns of seed deposition of three important riparian plant species (Acer negundo, Alnus incana, and Betula fontinalis) in relation to channel form and hydrologic processes both in field and flume experiments. Understanding the mechanisms of seed delivery along natural and regulated streams and the interactions between the timing of seed release, and the availability of exposed seedbeds at appropriate times, will provide a solid scientific basis for modifying flow regimes at certain times of the year to facilitate the reproductive success of key riparian species.
