Malham Tarn is situated in the Yorkshire Dales, a national park in the Yorkshire Pennines. It lies approximately 25 miles (40 km) north-west of Bradford and about 2.5 miles (4.0 km) north of the nearest settlement, Malham.
At 377 metres (1,237 ft) above sea level it is the highest lake in England and the highest marl lake in Great Britain. The lake is one of only eight upland alkaline lakes in Europe having a pH between 8.0 and 8.6 The catchment area of the lake is 600 hectares (6.0 km2; 2.3 sq mi) and the main inflow is a stream at the lake's north-west corner. The lake is 4.4 metres (14 ft) at its deepest, with an average depth of 2.4 metres (7.9 ft) and the surface area is 62 hectares (0.62 km2; 0.24 sq mi). It takes approximately 11 weeks for water to leave the lake after it has entered. The primary outflow is a small stream at the southern end of the lake. The outflow stream goes underground after approximately 500 metres (1,600 ft) before emerging downstream of Malham Cove as a source of the River Aire.