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A strategic programme for NERC Lowland catchment research
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Making space for water, plants & fish

Building development and flood protection works are increasingly disconnecting rivers from their banks and floodplains. However, with sympathetic management, river channels and their margins can form important ribbons of biodiversity across our lowland landscapes. In such systems, water, sediment and plants interact to provide complex, ever-changing habitats for animal life, including fish. LOCAR scientists have been assessing the role of river margins, plants and side channels in maintaining a healthy river ecology.

Rivers continually change as they erode and deposit sediment, and plants play a crucial role in engineering their ever-changing form. River bank trees such as willow and alder, water edge plants such as reedmace, and submerged plants such as water crowfoot are all common along lowland rivers. When inundated, these plants slow the current causing sand, silt and organic material to settle out of the water. Sediment accumulations create new landforms, new seed banks, a seed bed for the growth of new plants and sites for decomposition. Winter floods are vital for dispersing seeds from aquatic and riverside plants. Twice as many species are deposited across river banks by winter floods than during the low flows of summer.

Trees and other rigid plants on the river margins help to drive progressive changes in rivers by trapping and reinforcing sediment. Working in the Rivers Frome and Tern LOCAR scientists found that flexible water plants, such as water crowfoot, cause annual cycles of sediment trapping as they grow in the spring and decay in the winter. Water crowfoot also provides habitat for the larvae of filterfeeding insects and a settling area for their waste. As a result, sediment accumulations around these plants are hotspots for processing organic matter. During summer, aquatic plants are also very important for raising river levels, shading cool patches of slow-flowing water and inducing faster currents between plants which clean river bed gravels.  More about the influence of vegetation on our rivers

These processes are important in both the main river and in side channels such as natural tributaries, mill streams or drainage ditches. Contrasts in local habitat conditions along and between side channels are crucial in maintaining fish populations and their diversity. Using electronic tags, LOCAR researchers found that movements between the main river and side channels are an important part of life for many river fish. Mill streams and small tributaries provide important spawning sites for dace, while the slow-flowing drainage ditches are preferred by pike. In the high flows of winter, many types of fish use side channels as refuges from the main channel.  More about how fish use our rivers

These results are important for river managers. The diversity of river land-forms, flow velocities, patches of sediment and plant communities along lowland rivers and streams is heavily dependent upon engineering by plants. Sensitive vegetation management is critical for habitat complexity, biodiversity and organic matter processing within lowland rivers. Managers need to give space for riverbank plant communities and to maintain a balance between plant growth and channel choking where supplies of fine sediment and nutrients are high. Moreover, it is not just the main channel which needs sensitive management. Careful vegetation removal and desilting at the mouths of side channels can make them more accessible to fish and their food sources; but highly engineered, heavily dredged ditches and other side channels are of very low value to fish.