THE INFLUENCE OF GEOMORPHOLOGY AND THE SEDIMENTARY REGIME ON DISTURBANCE AND SUCCESSIONAL PROCESSES IN RIPARIAN ZONES OF THE LAKE ERIE GORGES ECOREGION OF WESTERN NEW YORK STATE

Riparian corridors are among the most diverse of ecosystems, owing largely to a complex interplay of hydrologic, geologic, and ecological factors (including processes of disturbance and/or vegetational succession). A conceptual model is proposed here, derived from an extensive study of channel morphology, sedimentary regime, and riparian forest ecology within eight middle-order river reaches of the Lake Erie Gorges (LEG) ecoregion of western New York State. Landforms stable enough to support forest development are associated only with either coarse clastic or fine clay/silt sediments, erosion resistant via particle mass or cohesiveness, respectively. Coarse sediment bed rivers possess stable bar and island forms deposited laterally and episodically by very high flow events. Fine sediment rivers are more often meandering single thread channels, with most bank accretion being vertical. Hydrology can influence riparian ecology of either channel type directly – often by flooding – or indirectly, by shaping the geomorphic template on which vegetation develops. Hydrologic alteration of geomorphology must necessarily alter (or remove) associated vegetation. However, and conversely, vegetation can be disturbed by flooding with underlying geomorphology remaining unchanged. When susceptibility of both vegetation and geomorphology to hydrologic influence eventually become nil, a landform has become effectively “terrestrial.” This study suggests that coarse sediment fluvial surfaces, often high above the channel and draining quickly, can be decoupled from direct hydrologic influence in as little as 20 years, yielding landform mosaics of various stages of ecological succession, even including intermediate- and late-seral “upland” species such as northern red oak, sugar maple, and eastern hemlock. In contrast, fine sediment banks drain slowly and can be inundated for long periods, yielding bands of flood-tolerant species such as silver maple and various willows. Riparian ecology remains tied to the hydrologic regime here for many decades. Both of these scenarios can play out even along the same river (e.g. moving from friable shale bedrock and glacial outwash zones to downstream lake plains within the LEG), further enhancing geomorphic and ecological diversity of the riparian corridor.