In working on the recent GeoCurrents post on dams, I was astounded to learn that the world’s second largest human-created lake, by surface area, is Smallwood Reservoir in northeastern Canada, which covers 6,527 km2 (2,520 sq mi) of land. This massive lake was created not by an individual dam, but rather by 88 dikes that together stretch over 64 km (40 mi). An idea of the size of the Smallwood can be gathered from the map to the left that overlays the lake on the Tokyo area; this image is found on MAPfrappe, a site devoted to “Google Maps mashups that let anyone outline an area on one Google Map, and compare that area on another Google Map.”
Smallwood Reservoir is also of interest because it created one of the world’s largest dry waterfalls. Before completion of the dikes in 1970, the Churchill River flowed over Churchill Falls, a magnificent cascade entailing a drop of 245 ft (75 m). Today, according to the Wikipedia, “water flows down the falls less than once a decade, during spring thaw or periods of exceptional rains.” The Churchill, River and Falls, were re-named after Winston Churchill in 1965; previously, it was known as the Hamilton River, after an early governor of Newfoundland.
Dry waterfalls are not uncommon, as noted in the World of Waterfalls website. Most were desiccated by hydroelectric facilities. Sometime the creation of a dry falls is strictly temporary; for a several month period in 1969, for example, the U.S. side of Niagara Falls was deprived of water to allow engineering work in the area (photo above).
The world’s largest natural dry falls is the aptly named Dry Falls of eastern Washington in the U.S., which resulted from the catastrophic draining of glacial Lake Missoula, which occurred several times during the last ice age. The Wikipedia article provides a good description:
Dry Falls is a 3.5 mile long scalloped precipice in central Washington, on the opposite side of the Upper Grand Coulee from the Columbia River, and at the head of the Lower Grand Coulee. Ten times the size of Niagara, Dry Falls is thought to be the greatest known waterfall that ever existed. According to the current geological model, catastrophic flooding channeled water at 65 miles per hour through the Upper Grand Coulee and over this 400-foot (120 m) rock face at the end of the last ice age. At this time, it is estimated that the flow of the falls was ten times the current flow of all the rivers in the world combined.
In contrast, North Carolina’s Dry Falls is not a dry falls at all, but rather a normal waterfall with enough of an overhang that one can usually walk under the cascade and remain dry.