Lower Rideau Lake

Big Rideau Lake
Sunset on the lake
Big Rideau Lake Location in southern Ontario
Location	Lanark County, United Counties of Leeds and Grenville, Ontario
Coordinates	44°45′05″N 76°12′50″W / 44.75139°N 76.21389°W / 44.75139; -76.21389
Part of	Saint Lawrence River drainage basin
Primary inflows	Black Creek, Rideau River
Primary outflows	Rideau River
Catchment area	407 square kilometres (157 sq mi)
Basin countries	Canada
Max. length	32 kilometres (20 mi)
Max. width	6 kilometres (3.7 mi)
Max. depth	360 feet (110 m)
Surface elevation	123.1 metres (403 ft 10 in)
Settlements	Portland, Rideau Ferry

Big Rideau Lake is a lake in the municipalities of Tay Valley and Drummond/North Elmsley, Lanark County and Rideau Lakes, United Counties of Leeds and Grenville in Eastern Ontario, Canada. The lake is on the border between the two counties, 72 kilometres (45 mi) to the southwest of Ottawa. It is 32 kilometres (20 mi) long and is 6 kilometres (3.7 mi) wide, is much narrower at its northeastern end than at its southwestern end, and is part of the Saint Lawrence River drainage basin. It is the largest lake on the Rideau Canal, which was designated a UNESCO world heritage site in 2007.

The primary inflow, at the southwest, is the Rideau River/Rideau Canal from Upper Rideau Lake via The Narrows channel and the Narrows lock and control structures. A second primary inflow is Black Creek at the west. The primary outflow, at the north, is also the Rideau River/Rideau Canal which leads via a channel to Lower Rideau Lake. The Rideau River and Canal flow to the Ottawa River and then to the Saint Lawrence River.

The small community of Rideau Ferry is located at the junction between the Big Rideau and Lower Rideau Lake, at the northeastern end of the lake. The community of Portland is located along the southern shore of the lake.

The shoreline of the Big Rideau Lake is lined with hundreds of cottages and houses. A public recreation area and campground, Murphys Point Provincial Park, is located along a substantial stretch of the shoreline on the northern side of the lake.