The lake, which was originally called the Yeo reservoir, covers 440 acres (180 ha). The River Yeo rises in the centre of Compton Martin village, from there it flows past the village of Ubley and enters Blagdon Lake. From the lake the river flows south of Wrington and Iwood, where there were once a series of watermills along its banks. Its route then takes it around the northern outskirts of Congresbury, and across the North Somerset Levels roughly parallel to the A370 road, past the site of a Roman villa before crossing under the M5 motorway and emptying into the Severn Estuary in Woodspring Bay, downstream from Clevedon and west of the village of Kingston Seymour. The watershed catchment area of the reservoir is 2,144 hectares (5,300 acres).
Aerial photograph showing Blagdon Lake in the foreground and Chew Valley Lake in the distance.
The construction of the reservoir was approved by acts of parliament in 1888 and 1889 as part of a plan to provide drinking water to Bristol, supplementing Barrow Gurney Reservoirs and the “line of works” pipe which carries water from the Mendip Hills. Construction required the provision of sewers to local villages to avoid contamination of the reservoir. The work was designed by Charles Hawksley. In conjunction with the construction work the Wrington Vale Light Railway was established connecting Blagdon with Congresbury on the Cheddar Valley Line and Yatton on the main Bristol to Exeter line. In addition to carrying construction materials for the reservoir and pumping station it also provided passenger services. Later the railway was used to bring coal for the pumps. Passenger traffic ceased in 1931 and the line to Blagdon closed completely in 1950.
The lake is relatively shallow, with an average depth of 14 ft (4.3 m) and only 42 ft (13 m) at its deepest point near the dam at the west end of the lake. The eastern end is the shallowest where the River Yeo enters. It is a hard water area with high mineral content that is vulnerable to eutrophication in the summer, that can cause algal blooms, with transient water layers. As a result, a research programme has been established to observe and measure the limnology of the lake. When full it contains 8,456,000,000 litres (1.860×109 imp gal; 2.234×109 US gal) and supplies 9,547,000,000 litres (2.100×109 imp gal; 2.522×109 US gal) of water each year. Water can also be taken from its larger neighbour Chew Valley Lake to balance the supply.