<Li> A shrunken lake is a lake which has drastically decreased in size over geological time . Lake Agassiz, which once covered much of central North America, is a good example of a shrunken lake . Two notable remnants of this lake are Lake Winnipeg and Lake Winnipegosis . </Li> <P> Paleolakes are of scientific and economic importance . For example, Quaternary paleolakes in semidesert basins are important for two reasons . First, they played an extremely significant, if transient, role in shaping the floors and piedmonts of many basins . Finally, their sediments contain enormous quantities of geologic and paleontologic information concerning past environments . In addition, the organic - rich deposits of pre-Quaternary paleolakes are important either for the thick deposits of oil shale and shale gas that they contain or as source rocks of petroleum and natural gas . Although of significantly less economic importance, strata deposited along the shore of paleolakes sometimes contain coal seams . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2013) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2013) (Learn how and when to remove this template message) </Td> </Tr>

What is the definition of a lake in the northern hemisphere