Gravel chub silt and debris. They are more often found in areas with limited macrophytes and larger root aquatic plants, algae and moss. Because of their need of a specialized clean habitats, the gravel chub is especially vulnerable to various forms of pollution, especially from agriculture practices that create erosion and excessive silt. In past sampling surveys the gravel chub had been found as far north in the Upper Iowa River as Le Roy, Minnesota, but because of excessive amounts of sewage, pesticides and sedimentation of rocks and sand, this is no longer the case. The presence of gravel chubs in a
Wivenhoe Gravel Pit establishing the previous course of the Thames. Two worked flints may be evidence of occupation by Homo heidelbergensis around half a million years ago. The pit, which is now water filled, is in a field off Brightlingsea Road opposite Broad Lanes. Wivenhoe Gravel Pit Wivenhoe Gravel Pit is a 2.1 hectare geological Site of Special Scientific Interest north of Wivenhoe in Essex. It is a Geological Conservation Review site. The site is the type locality for the Wivenhoe Gravel, which was laid down by the River Thames before it was diverted south to its present course by the Anglian glaciation