recovery project was set up by the Chilterns Chalk Streams Project and the Environment Agency, combining mink control with habitat improvement to try to halt this decline. In 2005 a survey found that the water vole population had recovered to 18% of its 2001 levels. A less positive story is that of the native white-clawed crayfish, which has suffered at the hands of another American import, the signal crayfish. Originally imported by the aquaculture industry, the signal crayfish was seen as a way for trout farms to diversify and exploit new markets. However, due to their expert, escapologist nature, signal crayfish soon escaped into surrounding river systems and quickly spread throughout the majority of the UK's southern river system. Signal crayfish have carried crayfish plague, to which they are resistant, but the native white-clawed crayfish is not. The result has been the gradual eradication of native crayfish from the UK's rivers. In the case of the Chess, white-clawed crayfish have been extinct since the mid-1990s. The Chess is also home to a number of invasive alien plant species including Japanese knotweed, Himalayan balsam and Orange balsam. Not all imports have quite such a catastrophic effect. Another escapee into the Chess from fish farms and trout fisheries is the rainbow trout, thought to have been introduced to the Chess in the early 20th century. They have since gone on to establish one of a handful of self-sustaining populations in the UK. Some of the flow of the Chess results from the outflow of treated sewage from Chesham Sewage Treatment Works, which is material during dry conditions. During heavy rainfall, overflow at the works results in untreated sewage being released to the river. In 2014, a release persisting 5 months occurred, with substantial effects on river quality. Upgrade works to address the increased flow into the works have subsequently been completed. Chess cress Historically, the clear chalk stream water of the River Chess, together with the fertile land, was ideal for growing watercress, and this industry which flourished in both Chesham and Rickmansworth in the Victorian era supplied London, being transported on the newly constructed Metropolitan Railway. Today the only working watercress beds are at Moor Lane, Sarratt. In 2014, persistent overflows from Chesham Sewage Treatment Works forced the watercress farm to cease sales, and to continue operation the farm now uses well water. Chess Valley Walk The Chess Valley Walk is