The effects of seasonal changes to in-stream vegetation cover on patterns of flow and accumulation of sediment

Abstract

In-stream macrophytes are typically abundant in nutrient-rich chalk streams during the spring and summer months and modify the in-stream environment by altering river flows and trapping sediments. We present results from an inter-disciplinary study of two river reaches in the River Frome catchment, Dorset (UK). The investigation focused on how Ranunculus (water crowfoot), the dominant submerged macrophyte in the study reaches, modified patterns of flow and sediment deposition. Measurements were taken on a monthly basis throughout 2003 to determine seasonal patterns in macrophyte cover, associated changes in the distributions of flow velocities and the character and amount of accumulated fine sediment within stands of Ranunculus. Maximum in-stream cover of macrophytes exceeded 70% at both sites. Flow velocities were less than 0.1 m s − 1 within the stands of Ranunculus and accelerated to 0.8 m s − 1 outside the stands. During the early stages of the growth of Ranunculus, fine sediment mostly accumulated within the upstream section of the plant but the area of fine sediment accumulation extended into the downstream trailing section of the plant later in the growing season. The fine sediment accumulations were dominated by sand (63–1000 μm) with silts and clays (0.37–63 μm) comprising < 10% by volume. The content of organic matter in the accumulated sediments varied within stands, between reaches and through the growing season with values ranging between 9 and 105 mg g − 1 dry weight. At the reach scale the two sites exhibited different growth forms of Ranunculus which created distinctive patterns of flow and fine sediment deposition.