Dim ond i gyforgronfeydd mawr y mae Deddf 1975 yn berthnasol ar hyn o bryd – hynny yw, cronfeydd dŵr sydd wedi’u cynllunio i ddal dros 25,000 metr ciwbig o ddŵr uwchlaw lefel naturiol y ddaear gerllaw. Pennwyd y trothwy hwn yn sgil methiannau cronfeydd dŵr o’r maint hwn yn yr 1920au a achosodd farwolaethau ac a ysgogodd Ddeddf 1930. Ar hyn o bryd, mae 1,925 o gyforgronfeydd mawr yn Lloegr a 201 yng Nghymru.
The 1975 Act currently applies only to large raised reservoirs – that is, a reservoir which is designed to hold or capable of holding more than 25,000 cubic metres of water above the natural level of the adjoining ground. This threshold was set on the basis of failures of reservoirs of this size in the 1920s which caused loss of life and prompted the introduction of a 1930 Act. There are currently 1,925 large raised reservoirs in England and 201 in Wales.