Geography of Kent and Dungeness have been formed by accumulation of alluvium. Kent's principal river, the River Medway, rises near Edenbridge and flows some 25 miles (40 km) eastwards to a point near Maidstone, when it turns north. Here it breaks through the North Downs at Rochester before joining the River Thames as its final tributary near Sheerness. The river is tidal as far as Allington lock, but in earlier times cargo-carrying vessels reached as far upstream as Tonbridge. The Medway has captured the head waters of other rivers such as the River Darent. There are other rivers in Kent, most notably the
St. Jones River is fed by Fork Branch, Penrose Branch, and Maidstone Branch. From Silver Lake, the St. Jones River flows generally southeastwardly, along the east side of downtown Dover and past Legislative Hall, the Capital Square/Legislative Mall, and the Dover Air Force Base to Bowers, where it flows into Delaware Bay, approximately north of the mouth of the Murderkill River. The lower course of the river southeast of Dover is surrounded by brackish marshes and salt marshes, open water habitats, and wetlands. A portion of the lower river, along with nearby Blackbird Creek, have received federal protection as the Delaware National Estuarine