20 m3/s, and some are run with less than 1 m3/s (usually these involve skidding the kayak down steep rockslides and small waterfalls). In Scotland there are also a few bigger volume (up to about 50 m3/s) rivers. Like many places in Europe, England, Scotland and Wales have been subject to centuries of rivers getting dammed, weired, or diverted in ways that suit agriculture or land development and the natural flow has been altered heavily in many regions; there are many orphan dams or sites intended for grist mills that no longer have any function but obstruct the water. Some areas of the island Great Britain occupies has enough gradient to create rapids and waterfalls, but unfortunately ages of damming prevents the full volume of water and snowmelt from reaching the lower reaches: Clywedog Dam holds back approximately 50,0000 megalitres of water alone on the River Severn, and Parliament granted weirs and locks to be placed upon that river multiple times during the 19th century, some of which are still used. Scotland is famous for its rocky north, but many rivers have been dammed in Scotland over the course of the 20th century to create hydroelectric power, with the trend still continuing. Almost all runs in England and Wales need recent rain to be at a paddleable level, and many can only be run immediately after heavy rain. In Scotland, some bigger rivers can be run for weeks after rain although as with the rest of the country, most need recent wet weather. The paddling season is year-round but the rivers are more often runnable in winter (the wettest months of the year being December and January). Exceptions to this include rivers which have artificially maintained flows from reservoirs. On these rivers, flow may increase in dry weather as more water is released. The Afon Tryweryn is one example in Wales. Most runs offer only a few kilometres of whitewater; often several rivers can be run on a wet day. Some rivers consist of only a single rapid. Only a few rivers (such as the Findhorn and Spean in the Scottish Highlands) have more than a days' worth of paddling, and most of this tends to be grade III or less. The River Dart excepted, there is no natural whitewater in the (mainly flat) south and east of England. Here whitewater paddlers often go playboating at man made weirs. Hurley