Ward (1989) introduced the concept of lotic systems in four dimensions – the fourth dimension of course being time. This concept is readily expanded into the riparian zone, within which geomorphic and ecological spatio-temporal dynamics often represent disturbance responses (secondary succession) vs. community development on new substrata (primary succession). Vegetation composition and age structure offer an ecological timeline. Topography and remotely-sensed imagery reconstruct geomorphic history, although the latter is necessarily limited in temporal scope. Ground Penetrating Radar (GPR) is allowing temporal expansion of geomorphic reconstruction by revealing subsurface features that may predate the oldest imagery. We are quantitating fluvial landform and riparian forest dynamics in the Zoar Valley Canyon of 6th-order Cattaraugus Creek in western New York State, to reconstruct the ecological history of this dynamic and very nearly anthropogenically unaltered system. Stand composition and age are previously described, and present a primary successional chronosequence of >300 years. We have catalogued aerial imagery to 1929, through which we are reconstructing channel and terrace morphology and interpolating rates of change. We are using GPR to extend geomorphic reconstruction temporally beyond that offered by surface imagery.
Results/Conclusions
Cattaraugus Creek through Zoar Valley follows a meandering and occasionally island-braided channel, with large episodically deposited bar forms on which forests develop via primary succession. With a few exceptions, terraces have consolidated and the channel simplified since 1929. Channel change has been primarily via gradual migration (a few meters per year) rather than by abrupt shifts. GPR has been valuable in resolving apparent disconnects between landform imagery and forest ecology. One particularly well imaged transect reveals a buried riverbank traversing a terrace fully contiguous in 1929, which clearly demarcates a pioneering cottonwood/sycamore stand (~130 year age) from a sugar maple-dominated mesophytic forest that exceeds 200 years. Under the oldest terraces – geomorphically contiguous and uniformly late in succession – GPR still images old river banks and channel swales that reveal long-buried landform development. We have hypothesized that the channel simplification evident in aerial imagery may partly reflect hydrologic changes associated with widespread reversion of marginal farmland to second-growth within the watershed. We are hoping that increasingly detailed remotely-sensed and subsurface imagery, combined with ecological history offered by forest stands, will allow reconstruction of ecosystem dynamics far enough into the past to resolve such questions.