Ecological functions of restored gravel bars, the Trinity River, California

Abstract

Gravel augmentation has been increasingly used in sediment-limited systems in regulated channels as a means of creating morphological changes that beneficially affect the functioning of downstream ecosystems. Despite this trend, there have been few empirical studies to quantify these effects in relation to the morphology of gravel bars, especially in terms of riverine material exchanges such as heat and organic matter. We conducted field-based hydro-geomorphological observation of different mechanisms of gravel bar restoration in the downstream of Trinity Dam, California: a medial and a point bar deposited fluvially during high-flow gravel injection, an additional point bar created by direct placement of gravel, and an island created by side channel excavation. We measured water temperature, suspended particulate organic matter (S-POM) concentration, hydraulic gradients and shallow water width under base flow conditions along the perimeter of the gravel features. We then assessed water temperature modulation derived from hyporheic exchanges and S-POM retention of the gravel features, comparing the functions of the dynamically-constructed medial and point bars to those of the mechanically-constructed island and point bar. Diurnal water temperature fluctuations showed a notable thermal heterogeneity including cooling (1.5–3.1 °C) during summer peak temperatures, buffering in amplitude (1.2–4.0 °C) and lagging in phase (0.3–14.5 h), especially in the bar-tails and alcoves of the gravel bars. All of the gravel features reduced S-POM concentration at baseflow, showing the highest retention efficiency in the medial bar. In addition, the fluvially formed the medial and point bars had higher hydraulic gradients and wider shallow waters than the constructed features. Our results indicate that gravel bar restoration can increase hyporheic exchange and S-POM retention by increasing hydraulic gradients at baseflow, refreshing bed materials to enhance substrate permeability, and elongating the wetted boundary length in shallow waters. Our study results suggest that mechanically created in-channel geomorphic features combined with coarse sediment augmentation can increase channel complexity, driving hyporheic flows and increasing S-POM retention, thus ultimately resulting in thermal heterogeneity and food availability along gravel bed channels.