Kent coastline of Kent is continuously changing, due to tectonic uplift and coastal erosion. Until about 960, the Isle of Thanet was an island, separated by the Wantsum channel, formed around a deposit of chalk; over time, the channels silted up with alluvium. Similarly Romney Marsh and Dungeness have been formed by accumulation of alluvium. Kent's principal river, the River Medway, rises near East Grinstead in Sussex and flows eastwards to Maidstone. Here it turns north and breaks through the North Downs at Rochester, then joins the estuary of the River Thames as its final tributary near Sheerness. The Medway is
Zumbro River The South Fork Zumbro River, long, rises about east of Hayfield in southern Dodge County and flows generally eastwardly into Olmsted County, where it turns northward at Rochester and flows into southwestern Wabasha County. The South Fork's course through Rochester has been channelized as part of a flood control project. It is dammed in Wabasha County, by the Lake Zumbro Hydroelectric Generating Plant to form Lake Zumbro. The Middle Fork Zumbro River, long, rises in northeastern Steele County, about west of West Concord and flows generally eastwardly through northern Dodge, southwestern Goodhue and northeastern Olmsted counties, past Pine Island and