The two reservoirs were built in 1959 as part of the Tophill Low Water Treatment Works. The scheme was designed to treat and hold water taken from the River Hull for use as drinking water in the East Hull and Holderness area. Both the reservoirs were constructed with vertical concrete walls with the natural boulder clay of the area forming the bottom. `D` Reservoir is 4 metres deep covering an area of 24ha whilst `O` Reservoir is 8 metres deep covering an area of 10ha. During the late winter to autumn the algae from the nutrient rich river water starts to proliferate, this results in numbers of invertebrates like snails, water fleas and other insects increasing which inturn becomes food for the birds. Due to their attractiveness to birds in 1989 English Nature designated to two reservoirs as Sites of Special Scientific Interest, this was mainly due to more than 1% of the British wintering populations of Gadwall, Shoveler and Tufted Duck using the reservoirs as wintering sites along with locally important figures of Goldeneye, Great Crested Grebe, Mallard, Pochard, Teal and Wigeon.

>> Species
Spring - From mid to late March the first Sand Martins will appear feeding on
the freshly emergent midges. These are closely followed by Swallows and House Martins early in April and then Swifts in early May. When the weather
is cold and wet and flying insects are in short supply large numbers of the above four species can be seen sometimes numbering into the thousands
as they come into feed from the surrounding area. Terns can generally be seen on passage over the reservoirs with Common, Arctic and Black Terns the main species seen. Other birds attracted to the reservoirs are Yellow Wagtails, Pied Wagtail, White Wagtails and Common Sandpipers which feed on the walls surrounding the reservoirs.

Summer - Black-necked Grebes are a possibility during the summers months and individuals may stay on to moult into winter plumage. During July and August large numbers of Tufted Duck, recently as many as 1000, use the reservoirs to moult their flight feathers.

Winter - Due to the `D` Reservoir being shallower and covering a larger surface area than `O` Reservoir this tends to hold the largest number of wildfowl. As well as those mentioned earlier there is also the possibility of seeing scarcer visitors at Tophill including Goosander, Smew, Common Scoter, Long-tailed Duck, Scaup as well as the occasional Bewick`s and Whooper Swan. The reservoirs also attract upto 100 wintering Ruddy Duck.

Non-Breeding Season - During the non breeding season `D` Reservoir holds a large gull roost. This is best seen in the evenings and with all the species involved can number in excess of 30,000 birds.