INSIGHTS FROM COMPARING VARIABILITY IN RATES OF TOPOGRAPHIC CHANGE OVER TENS TO HUNDREDS OF YEARS IN A CHANNEL NETWORK

Restoration and management of natural resources at the landscape scale require an understanding of rates of modern processes and their departures from geologic norms. Quantifying surface change with methods that integrate over timescales much longer than any possible anthropogenic forcing is one approach that can be used to establish metrics for background geomorphic behavior. In order to accurately assess the significance of any observed difference, it is imperative to know the effects that integration over disparate timescales has on a set of measurable surface changes. This includes the need to assess changes in variability through time.

Here, we present a collection of 381 estimates for rates of topographic change collected over 2 km of a channel network adjacent to the Apalachicola River, Florida. Exposure of tree roots in this minimally disturbed landscape provides estimates of rates of topographic evolution over durations of decades to centuries. The collection represents estimates from 1/3 of all trees greater than 2 inches in diameter at breast height. Comparison of rates as a function of timescale shows that mean rates appear to remain steady over the recent past when both erosion and non-erosion rates are included in the collection. However, when only erosion rates are compared, then the mean rates appear to decrease as a function of timescale. The magnitude of this effect could be large and appears to scale with the square root of the timescale. Similarly, variance of topographic change increases linearly with timescale for the biased collection. All investigations that intend to compare historic and modern rates should account for the possible bias associated with only including erosion rates from an evolving channel network.