A northerly flowing river, the Avon River flows rises on South Mountain, southwest of the town of Windsor. Its meander length is . Near the rural community of Martock, the river enters a broad glacial river valley forming a ria) where it becomes tidal, creating an estuary for its remaining route to the Minas Basin several kilometres downriver from the town of Hantsport.
The Avon River Causeway replaced an existing road bridge upriver from town and also resulted in the rerouting of the Dominion Atlantic Railway's Halifax-Yarmouth main line which used to run through Windsor's downtown, crossing the river on a bridge parallel to the road bridge immediately upriver from the town.
The causeway controls the Avon River's discharge and the incoming tidal waters of the Minas Basin through a series of flood control gates which are intended to regulate the river's flowage to prevent flooding of agricultural lands upriver near Martock. The section of the Avon River upriver of the causeway along the Windsor waterfront is now the freshwater Pesaquid Lake.
The construction of the causeway has dramatically affected the Avon River downstream from Windsor, with large parts of the once-navigable river now being obstructed by large mud flats and vegetation, owing to the lack of tidal exchange and freshwater discharge. The nature writer Harry Thurston has noted, "Almost before the last stone was put in place, sediment began to accumulate to an alarming rate - 5 to 14 centimetres per month. Within seven years, a four metre high island of silt formed on the seaward side of the causeway; and the effects were felt 20 kilometres downstream, where two metres of mud impaired navigation at Hantsport.
Researchers soon discovered that the mud flats had become a biological desert, devoid of life, as the sediments were too soft to support organisms. The need to monitor and assess the impacts of such changes gave impetus to a greater interest in Bay of Fundy ecosystem and the ultimate establishment of the Acadia Centre for Estuarine Research in Wolfville.
Recently announced plans for the expansion of Highway 101 between the Halifax Regional Municipality and the eastern end of the Annapolis Valley have raised concerns about maintaining the Avon River causeway. An environmental lobby group, Friends of the Avon River (FAR), has called for studies into the possibility of removing the causeway entirely and carrying the expressway and railway line on a new bridge, allowing the natural flow of the river course to be reestablished. FAR has also recently begun to oppose the expansion of gypsum quarries within the river's watershed because of threats to river life, citing the use de-watering runoff from the quarries.
A decision in August 2007 by the New Brunswick government to remove a similar causeway (constructed in 1968) blocking the Petitcodiac River, another tribuatary of the Bay of Fundy, to avoid facing charges under the federal Fisheries Act, is expected to have ramnifications for the Avon River causeway.