Авон өзені: географиясы, тарихы және экологиялық өзгерістері

NOTOC
The Avon River is a small river in central Nova Scotia, Canada. A northerly flowing river, the Avon River's flow rises at an elevation of 145 metres (475 ft.) at Card and Bag Lakes on the South Mountain, a distance of approximately 29 kilometres (18 miles) southwest of the town of Windsor. Its meander length is At Windsor Forks, the West Branch Avon River (the main tributary) and the Southwest Branch Avon add their flow to the Avon. The West Branch Avon River rises at an elevation of 175 metres (574 ft.) at Black River Lake. Both the flows of the Avon and the West Branch have been impeded by hydropower developments in the early twentieth century. 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 at Kempt Shore, several kilometres downriver from the town of Hantsport. Another tributary, the St. Croix River joins just below Windsor creating a total watershed of 1,306 square kilometres. Rivers flowing into the Avon include the Halfway, Herbert, Cogmagun, Kennetcook and St. Croix. The Avon River is evident on many very early maps of the region and by 1686 is shown, along with its drainage basin, on Jean Baptiste Louis Franquelin's map in great detail.

In 1970 the Avon River was completely obstructed by a rock and earth fill causeway immediately downstream from Windsor at its junction with the St. Croix River as part of the development of a controlled access expressway called Highway 101. 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 a 'man made' freshwater, Lake Pesaquid. 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." Dr. Michael Risk of the University of Guelph, researching the build up of a new mud flat on the seaward side of the causeway, "estimated that the bottom dwelling creatures at Windsor were experiencing a two thirds mortality rate", which led him to state the new formation was a "biological desert". The onset of the mud flat formation left the sediments in an initial state which 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 (ACER) in Wolfville. Contrasting the initial decline of bottom dwelling creatures, Dr. Graham Daborn of ACER found that zoo plankton "are more abundant than previously thought. They appear to be able to thrive on non living organic matter " and that this "abundance of creatures at the lower end of the food chain suggests a considerable nursery role for the inner Bay." This notion is supported by research work completed by Dr. Sherman Bleakney of Acadia University. A decision in August 2007 by the New Brunswick government to remove a similar causeway (constructed in 1968) blocking the Petitcodiac River, another tributary of the Bay of Fundy, to avoid facing charges under the federal Fisheries Act, is expected to have ramifications for the Avon River causeway.