Constraining the timescales of sediment sequestration associated with large woody debris using cosmogenic 7Be

Abstract

The beneficial ecogeomorphic functions associated with large woody debris (LWD) in fluvial environments are well documented and include positive sediment impacts such as channel margin sequestration, increased substrate heterogeneity, and decreased channel embeddedness, as well as numerous secondary benefits such as nutrient retention and increased habitat heterogeneity. Despite an extensive literature documenting such positive sediment attributes of LWD in forested channels, a quantitative analysis of in‐channel sediment storage times associated with channel obstructions has traditionally been difficult to assess. In this study along a 9 km stretch of the Ducktrap River in coastal Maine we present a novel application of fallout cosmogenic 7Be (t1/2 = 53 days) coupled with a constant initial activity (CIA) sediment aging model to quantitatively assess transitional bed load storage times in bars associated with in‐channel obstructions (LWD and boulders). We find that reach‐scale variability in unit stream power and LWD frequency affect sediment storage times, with transport‐limited reaches providing longer‐term sediment sequestration (generally > 100 days) associated with in‐channel obstructions than supply limited ones (<100 days). Estimates of sediment baraccumulation rates also varied between reaches from 0.2 g cm−2 d−1 in the supply limited reach to 0.7 g cm−2 d−1 in the transport‐limited reach. Last, greater frequency of sites, increased sediment volumes and storage times, and naturally viable recruitment mechanisms for LWD in forested channels document its superior ecogeomorphic function when compared to boulders in this study, even in the Ducktrap river, where twentieth century logging has greatly reduced the size, frequency, and geomorphic efficacy of in‐channel wood. This study has implications for channel restoration efforts and documents a novel application of 7Be and CIA methodology to constraining transitional bed load storage times in the fluvial environment.