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A strategic programme for NERC Lowland catchment research
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How do we judge the health of rivers?

It is no longer sufficient to maintain rivers free from pollution. River managers are now trying to maintain their rivers in an ecological state that is as close to pristine as possible.

Swan and cygents on a Chalk stream

Rivers reflect the environments that they drain. They are sensitive to all human activities in their catchments, to introductions of new plant and animal species, and to the impacts of climate change. For a long time the quality of rivers has been assessed by measuring a few chemical factors or by counting the numbers of key types of fish, invertebrates or plants in the rivers. Assessed on such criteria, the LOCAR rivers would seem to be of good quality. However, LOCAR scientists have identified significant changes taking place in stream beds which suggest that these rivers may be less healthy than previously thought.

Stream beds are highly variable. The bed is a mosaic of patches: some muddy, some sandy and some gravelly. Some patches support dense stands of plants which trap silt and support abundant insect life, particularly the larvae of simuliid blackflies, whose wastes contribute to the chemical exchanges in the bed. The patches differ in the way they allow water to penetrate into the bed or, at low flows, to emerge from the bed into the stream. These contrasts affect the mixing of surface water and groundwater, the time that water is held between the grains of material in the bed, and the rate of chemical exchanges.

In the River Lambourn, LOCAR research found that most of the important chemical and biological interactions between surface and groundwater take place in a thin layer rich in organic matter, just 10 to 20cm thick, at the top of the river bed sediments. In the flowing water column above this layer, and in the rising groundwater below, nitrate is abundant. But in the thin active layer nitrate is chemically reduced, with the oxygen atoms being replaced by hydrogen. The ammonium which is produced is actively taken up by growing aquatic plants, and gases such as nitrous oxide and methane are generated by microbes. More about sediments & nutrient budgets

Such processes also occur in the River Frome, where it has also been shown that methane finds its way from the poorly oxygenated sediments below plant beds to the atmosphere via the stems of the plants. Evidence from the conventional biological assessment of these rivers suggests they are of good quality, but it seems unlikely that the production of nitrous oxide and methane is really characteristic of ‘good ecological status’. Farming practices leave these rivers rich in sediments and nutrients, creating the conditions in which such gases are produced and leading to low birth and survival rates among fish species of importance to anglers, including salmon and trout.