The transport and retention of fine sediments in seasonally vegetated lowland streams

Abstract

The oversupply of fine sediment to lowland streams has been associated with the general degradation of their habitats with symptoms including increased turbidity, decreases in groundwater to surface water exchange and a decline in biodiversity and Salmonid fecundity. Aquatic macrophytes have been implicated as a factor in fine sediment problems, due to their high capacity for localised fine sediment retention caused by macrophyte patches decreasing channel flow velocities. However, there are still gaps in current knowledge relating to seasonal macrophyte growth and the cycling of fine sediment within lowland streams. The objectives of this interdisciplinary study were to: (1) investigate the influence of changing seasonal macrophyte cover on fine sediment deposition and storage at the reach scale; (2) analyse the impact of changing seasonal macrophyte cover on fine sediment transport at the reach scale, and (3) examine the temporal and spatial changes of fine sediment deposits, including particle characteristics within Ranunculus patches. Two chalk stream reaches within the Frome-Piddle catchment were chosen for a two year comparison of two similar streams. The distribution and particle characteristics of deposited fine sediment were analysed at the reach scale using monthly site surveys and fine sediment samples taken within Ranunculus patches at both sites. Seasonal reach scale releases of corn pollen as fine sediment analogues were used to investigate fine sediment particle transport through both reaches in relation to concurrent changes of macrophyte cover and channel porosity. Lastly, three Ranunculus patches were investigated at the patch-scale at both sites over an annual cycle. Investigations were conducted to analyse variations in the volume of deposited and retained fine sediment and particle characteristics within and between patches and within patches in relation to seasonal patch growth. Fine sediment deposits were found within in-channel macrophytes and marginal vegetation, with considerable deposits found within the margins at both sites but particularly within Nasturtium and Apium patches on the Bere Stream. Statistical differences were found in the depth of fine sediment within emergent and submergent Ranunculus patches at both sites. Fine sediment samples from Ranunculus patches were dominated by sand-sized (63-1000 μm) particles in both effective and absolute 3 fractions, with smaller volumes (<10 %) of silt and clay-sized particles. No correlation was found between the seasonal cover of aquatic macrophytes and corresponding values for the transport (FX) and loss (KP) of corn pollen particles. However, corn pollen particles in transport were found to decrease dramatically in number when a combination of factors occurred together. The main influencing factors were the seasonal peak in macrophyte cover, seasonally low channel discharge and high values of channel roughness represented by Manning’s N values. Estimated values of channel porosity were not associated with corn pollen transport and retention. Values of corn pollen transport distance (SP) and depositional velocity (Vdep) were not associated with macrophyte cover or channel porosity. Fine sediment was found to be generally distributed within the rooted area of Ranunculus patches, with fine sediment deposited in the tail section in the summer months. The head zone of the Ranunculus patches were found to possess lower D50 values for fine sediment particle size at both sites, other than this there was little differentiation between all of sections within patches. The results from this study suggest that further development of a holistic approach to aquatic macrophyte management is required within some lowland streams to promote a sustainable balance for fine sediment transport. Future management involving localised weed-cuts should address problems with fine sediment and flow conveyance while also being considerate to habitat ecology and biodiversity.