6.0. Channel / Habitat / Biodiversity AIMS: Increase connectivity with the floodplain, reduce impoundment and barriers to fish,
improve resilience to increasing temperatures, enhance habitats for wildlife and control invasive species.
6.1 What are the problems? Structure and Function Chalk streams have been historically modified to suit a variety of human purposes such as agriculture, fisheries, flood risk management and the development of watercress beds and mills. The various modifications have altered the natural character of the river and led to systems being potentially over-‐widened, over deepened, impounded and disconnected with the floodplain. Barriers to Fish In channel weirs and structures are generally linked to negative impacts on the physical and ecological condition of the river. In addition to altering the depth and velocity of flow, structures can also impact on the biological connectivity of the river by limiting fish and invertebrate passage. Riparian Management The Test and Itchen supports a thriving fisheries industry and the majority of the river is intensively managed to accommodate sport fisheries. Inappropriate vegetation management practices can impact on the diversity of riparian habitats, over widening of section and the rivers ability to restore it’s natural features. Water Temperature Climate change models forecast that average summer air temperature will rise by between 2 and 4 degrees celsius. Rivers and the freshwater ecosystems they support are particularly sensitive to changes in climate and water temperature. For example a rise in water temperature above 22 degrees celsius for more than seven consecutive days can be lethal for brown trout. Non-‐native Invasive Species Traditional chalk stream flora and fauna faces increasing pressure from a range of non-‐native invasive species that have taken hold within the catchment. These include Himalayan Balsam, American mink and most recently the American signal crayfish. 6.2 Where are the pressures? There are numerous sources of data and evidence available to show the assessment of ecological health and biological richness across the catchment. Designated sites are classified according to their condition and rivers and other aquatic ecosystems are assessed via a number of metrics, included in the WFD ecological assessments, some of which have been be mapped in Figures 28-‐33 to highlight where certain pressures are most significant.