Not all of the water removed from the river by the swallow holes is returned to the channel at Leatherhead. The chalk aquifer also feeds the springs at the southern end of Fetcham Mill Pond, which have never been known to run dry. A survey in March 1883 estimated that the Fetcham springs were producing about every day. A second survey in 1948 estimated that the same springs were yielding about a day. The water table in the chalk of the Wey Gap is significantly higher than might be expected from natural rainwater percolation alone. It has been suggested that a proportion of the excess water originates from the Mole Gap.