Surface run-off in the upper catchment of the Hazel Brook, especially from the large retail centre at Cribbs Causeway, sends a good deal of silt into the system, slowing the flow and creating a risk of flooding downstream in the Trym. This problem has now been partially alleviated by the construction of the Catbrain attenuation reservoir near Cribbs Causeway.[5] Measurements of pollution by the city council show the water to be relatively clean.[6][7]