Hydrology, sedimentology, and geomorphology as drivers of succession vs. flood disturbance within riparian forests of middle order streams of the Northeast

Background/Question/Methods

Composition of riparian forests typically represents varying influences of flood disturbance and/or primary succession. The Zoar Valley Canyon along 6^th order Cattaraugus Creek in western New York State displays predominantly successional dynamics, although most riparian sites previously studied in the East appear flood-driven. The main objective of this meta-analysis within the Lake Erie Gorges ecoregion of New York and Pennsylvania is to understand the contributions of geomorphology, hydrology, and sedimentology to a disturbance vs. succession continuum for riparian forests of middle order (4^th-6^th) streams of the Northeast. We are quantifying distribution, potential source, and grain size of sediments in 6 – 8 selected river corridors, and using georeferenced aerial and satellite imagery and ground penetrating radar to reveal channel and landform temporal dynamics. A 75-year record of USGS gauging stations is allowing reconstruction of flood regimes. Composition and structure of riparian forests are being catalogued within 10-m thru 30-m square survey quadrats, with stand ages estimated by increment coring. Study streams are generally free of anthropogenic alterations for a century or more, which typically represents the most changeable time period of riparian ecology within the region.

Results/Conclusions

Riparian forests in Zoar Valley, previously studied thoroughly, reflected primary succession driven by spatio-temporal patterns of establishment of very coarse-sediment (gravel-cobble) landforms deposited as punctuated events.  There is evidence that a similar trajectory also prevails on coarse-sediment aggradational landforms in the riparian zones of other Lake Erie Gorges streams, even some substantially smaller than Cattaraugus Creek. High forest diversity is often seen, especially where aging pioneers (willow, cottonwood) and young late-seral species (beech, maple) overlap. At the other end of our suggested continuum, inner margins of stable or incrementally growing meanders often support bands of woody vegetation of increasing age moving inland, possibly reflecting either (or both) deposition or flood effects. Such forests are typically dominated by pioneer species. The role of sediment size and type in these situations is not yet entirely clear, although fine cohesive sediments are suspected of yielding flood-driven community dynamics.  It is increasingly evident that within the Lake Erie Gorges ecoregion various combinations of a wide range of flood regimes and sedimentary environments combine to drive geomorphology directly, which then provides the physical template for patterns and processes in riparian forest ecology.