summer sun

The rivers are very low after the long spell of dry weather. Askrigg and Newbiggin Becks are completely dry in sections and upstream of Worton the main Ure is turning green with algae. These periods of low flow, coupled with algal blooms, create seasonal bottlenecks that limit populations of fish and flylife. Algae can soak up the dissolved oxygen during the night, as plants switch from photosynthesis to respiration. During the early hours the river can become depleted of oxygen to the extent that severe fish kills occur. When the algae dies back it smothers gravel habitat and depletes oxygen as it decomposes through microbial action.

Under natural conditions upland rivers would be oligotrophic, or low nutrient, relying on seasonal inputs of leaf litter and dying salmon after spawning. But in urbanised and agriculutral catchments these dynamics are changed through the inputs of human-derived nutrients. It doesn't take much phosphate to lead to eutrophication (the change from a lower to higher nutrient status). When this occurs the ecology of the river changes too. This can be through reduced populations of typical species or invasions of species pre-disposed to survive under the emerging conditions. In dales rivers populations of fish, including salmon and trout, are lower then expected suggesting issues of pollution. But barriers, such as weirs, also limit their populations showing that impacts on rivers are multiple.

At one time pockets of de-oxygenated water moved with the tide along the Humber estuary stopping upstream migration of spawning salmon and trout, and downstream migration of smolts (salmon and trout that undergo physiological changes enabling them to live in saltwater). With more stringent regulations on industry, and the decline of the UKs indutrial base, this has been reversed and migrating fish stand a chance of reaching their spawning grounds. The rivers trust is trying to improve the habitat of these upland rivers to ensure that spawning streams are able to support populations of salmon and trout fry. Providing shade to river banks through tree planting, preventing cattle and sheep accessing rivers and adding structure to stream habitats (for example through the inclusion of large woody debris) all offer habitat for salmon and trount young.