One of the largest lakes in Cumbria, Bassenthwaite is also the only "lake" - all the rest being meres, waters or tarns. It is fed by the River Derwent which flows across a three-mile alluvial plain from Derwent Water to the south. It is possible that the two lakes were once one, until erosion deposits raised the level of the lakebed to fill in the present gap.

Bassenthwaite is a very fertile lake and supports numerous species of fish, including the vendace which is only found in two locations in England. During the summer it is also home to ospreys, which have returned to breed here since 2001. Encouraged by the provision of a nesting platform and protected by wardens from the Lake District National Park, Forestry Commission and RSPB, the birds have successfully raised 13 chicks up to 2007.

Bassenthwaite, being surrounded by large areas of cultivable land is suffering from several issues which the Bassenthwaite Restoration Project is attempting to address. The principle of these are erosion off the fells leading to silting up of areas of the lake and pollution from agricultural fertilisers.

There are various locations around the lake where the shore is accessible, these can offer fine views of the fells around. Due to its size, overall views of Bassenthwaite are limited without some strenuous walking (e.g., up Skiddaw). However, there is a good view available from the road leading to Whinlatter Pass or from Surprise View beside Derwent Water.