UK ECN site. The River Esk rises on the uplands of the North York Moors National Park and is the only major river in the county of Yorkshire that drains directly into the North Sea. The catchment is sparsely populated without the pressures of industrialisation and urbanisation affecting other rivers in the Region. Open moorland characterises much of the catchment and is an important habitat for a wide variety of wildlife. Within the Esk valley there are six Sites of Special Scientific Interest (SSSIs), two of which extend south and cross the boundary of the Derwent catchment. The source of the Esk is upstream of Westerdale, where a series of small streams (becks) known as the Esklets merge to form the River Esk. Many of these moorland streams are affected by natural "flushes" of acidity, as well as iron run-off from natural ironstone strata and old mineral workings, making some of these becks an ochreous-orange colour after periods of rainfall. The combination of the two factors restrict the invertebrate fauna in these head-streams. The majority of the River Esk downstream of the Esklets has very good water quality, with the diverse invertebrate fauna dominated by mayflies, stoneflies, caddisflies and other pollution sensitive groups. This good water quality is also very important in sustaining other species such as salmon, sea trout, dippers and otters. The ECN site is at Briggswath (NGR: NZ 868 081), approximately 2km upstream of the tidal limit. At this point the river is approximately 15m wide, and in normal summer flows the depths vary between 20cm and 30cm.