A delta is a deposit of sediment at the mouth of a river that accumulates as the river flows into a standing body of water such as a lake or ocean. Because sediment tends to be rich in nutrients, deltas become fertile wetlands inhabited by diverse wildlife. Among the largest deltas in Canada are those of the Mackenzie and Saskatchewan rivers, as well as the Peace-Athabasca Delta (where the Peace, Athabasca and Birch rivers meet). Certain deltas offer advantageous access to natural resources and maritime transportation, but development projects are often controversial due to the ecological importance of these environments.
Attawapiskat River, 748 km long, formed by the confluence of the Pineimuta, Trading and Otoskwin rivers at Attawapiskat Lake, in northeastern Ontario, flows east, jogs north and runs east to the flatland by James Bay. Its drainage area is 50 200 km2 and its mean discharge 626 m3/s.
Churchill River, Labrador, 856 km long (to head of Ashuanipi River), issues from Ashuanipi Lake, drops 75 m over Churchill Falls, broadens into Winokapau Lake and runs east through a deep glacial gorge past Happy Valley-Goose Bay into Lake Melville, Hamilton Inlet, entering the Atlantic near Rigolet (pop 317).