Modelling at the catchment scale

Understanding how water moves across the land and what chemicals and sediments it delivers to rivers is vital to the understanding of river ecology. The rivers trust movement has played a key role in recent years by helping to decipher how catchment processes control river morphology and how morphology and water quality affect ecology.

Excessive fine sediment in upland rivers degrades river habitats by clogging up the spaces between the gravel. This reduces egg survival of Salmon and Trout and changes the macroinvertebrate communities by favouring organisms such as Chironimidae worms over Stonefly and Mayfly.

A number of rivers trusts including the Yorkshire Dales Rivers Trust have been using a modelling tool developed by Durham and Lancaster universities called SCIMAP (Sensitive Catchment Integrated Modelling and Analysis Platform). This provides detail on where fine sediment is likely to be delivered to a watercourse based on slope, landcover and its associated erosion risk and rainfall.

SCIMAP outputs show the average risk for in-stream fine sediment in any catchment being modelled with a number of risk classes either side of the average ranging from high risk (red) to low risk (green). Underneath this output is a second output in grey that highlights the land parcels of a catchment most likely to be the source of the sediment. From this it is easy to locate the red 'streams' and then look up stream for the most likely locations that are delivering the sediment. Some ground truthing is then necessary but once the risk has been established it is possible to enter negotiations for changing land management to methods that can reduce these inputs. Simple measures such as buffer strips or contour planting of trees slows surface run off allowing sediment to settle out before the water reaches a watercourse.

This has the benefit of reducing the degradation of rivers and streams and allows species composition to restore itself back to the natural community of the river type. It is these simple habitat measures, developed through catchment scale thinking, that will restore habitats to something like their potential. This kind of large scale thinking is constantly developing which makes working as part of a rivers trust an exciting and rewarding vocation.