The impact of plants on fine sediment storage within the active channels of gravel‐bed rivers: A preliminary assessment

The role of aquatic and riparian vegetation in driving morphodynamics of rivers is being increasingly recognized across all river types. Here, we focus on gravel-bed rivers, where the ability of vegetation to influence morphodynamics depends upon the retention and stabilization of predominantly sand and finer sediments to build landforms within the active channel. One aspect of such interactions among vegetation, river flows and transported sediments that has received little research attention is their contribution to within-channel storage of fine sediment as a potentially important component of the fine sediment budget of river reaches and catchments. In this article, we assemble some preliminary estimates of the fine sediments retained by vegetation across the active channels of four gravel-bed rivers (12 river reaches) representing a wide range in river planform (near-straight, meandering, braided), width (6–800 m), gradient (0.0008–0.004) and Q2 flood (2–1100 m3 s−1), and including reaches where submerged and emergent macrophytes and riparian trees and shrubs act as physical ecosystem engineers. Our results indicate that vegetation can retain sizeable quantities of fine sediment, ranging from 480 to >3000 kg m−2 in emergent vegetation-engineered landforms, equivalent to 20–1000 kg m−2 when averaged across the entire active channel area. Vegetation retains virtually all the fine sediments stored on the bed surface of these active channels, increasing from 78% in the lowest energy to 100% in the highest energy river reaches investigated, with major differences in the locations of these vegetated fine sediment stores according to river planform style and unit stream power. These preliminary estimates suggest that fine sediment retention and storage by vegetation is potentially an important component of within-channel sediment budgets as well as contributing to the hydrological, geomorphological and ecological functioning of rivers.