DECADAL TO CENTENNIAL EROSION RATES IN SEEPAGE CHANNELS OF THE FLORIDA PANHANDLE

Erosion of unlithified Plio-Pleistocene sands and muds from groundwater seepage along the Apalachicola Bluffs in Florida has resulted in hundreds of kilometers of channel networks. The growth of these features is intimately connected with the downstream channel processes which are largely controlled by the vegetation in the valley bottoms. We use exposure of tree roots in channels, over springs, and elsewhere on the valley floor to constrain rates of vertical denudation and knickpoint migration. This is done by relating the distance between the modern geomorphic surface and the top of the tree's root structure to the age of the tree. Because the erosive process could have started anytime since the birth of the tree, the age is a maximum age of onset and the rate is therefore a minimum denudation rate. Although this technique is not new, there are few occurrences in the literature relative to the ease of its implementation and low cost.

The modal value of 35 minimum, vertical erosion rate estimates is ~0.5cm/yr. In addition measured knickpoint migration was of the order of 1cm/yr. These rates are low given the complete lack of cohesion of the sandy substrate. However, the vegetation, both live and dead, imparts an important effective cohesion upon the channel bottom and margins. Specifically, the knickpoints are created and upheld by channel migration onto subsurface root structures. Fallen trees and fine scale rooting are also integral in maintaining the channels. Because of the inherent heterogeneity of the spatial distribution of tree roots over decadal and centennial timescales, the overall spatial variability in these erosion rates is very large relative to the mean rate.