It covers an area of 467 acres (1.89 km2), is 2 miles (3.2 km) long, 0.75 miles (1.2 km) at its widest point and 12 feet (3.7 m) at its deepest.[3][4] The average depth of the lake is 4 feet 11 inches (1.5 m) and the mere itself lies only 26 feet (8 m) above sea level.[5] The mere is fed by several small streams and a sluice gate at the eastern end of the mere controls the outflow, which travels only 0.68 miles (1.1 km) eastwards to the North Sea.[6][7]
A Royal Naval Air Service seaplane base was opened on the mere in September 1917.[12] Initially, numbers 248 and 251 squadrons operated from RNAS Hornsea Mere with a headquarters at Killingholme. No. 251 Squadron operated non-water based aircraft so did not actually fly from Hornsea despite being based there.[13]
The RNAS base was located on Kirkholme Nab, a small 6-acre (2.4 ha) peninsula that sticks out westwards from the eastern shore. The RNAS installed two Bessonneau hangars and two slipways from Kirkholme Nab which allowed a dozen seaplanes to be operated from the base.[14][12] Various flights flew from here in coastal protection and submarine attack operations. These flights were eventually grouped together to form No. 248 Squadron who were commanded by No. 79 (Operations) Wing (who also commanded No. 251 Squadron, hence their allocation at Hornsea Mere).[15]
A number of brick buildings left behind by the RAF in 1919 are still in use today, employed by the boatyard and the cafe which operate there.[16][17] Despite being vacated by the RAF, the mere and the former RNAS buildings were used as a starting point for aerial practice runs on the bombing range at RAF Cowden on the East Yorkshire coast.[18]
^Winn, Christopher (2010). I never knew that about Yorkshire. London: Ebury. p. 38. ISBN978-0-09-193313-5.
^Chrystal, Paul (2017). The Place Names of Yorkshire; Cities, Towns, Villages, Rivers and Dales, some Pubs too, in Praise of Yorkshire Ales (1 ed.). Catrine: Stenlake. p. 105. ISBN9781840337532.
^"Hornsea Mere". environment.data.gov.uk. Retrieved 1 November 2018.