Page 164 - Sedimentology and Stratigraphy

10

Lakes

Lakes form where there is a supply of water to a topographic low on the land surface.
They are fed mainly by rivers and lose water by flow out into a river and/or evaporation
from the surface. The balance between inflow and outflow and the rate at which evapora-
tion occurs control the level of water in the lake and the water chemistry. Under condi-
tions of high inflow the water level in the lake may be constant, governed by the spill point
of the outflow, and the water remains fresh. Low water input coupled with high evapora-
tion rates in an enclosed basin results in the concentration of dissolved ions, which may
be precipitated as evaporites in a perennial saline lake or when an ephemeral lake dries
out. Lakes are therefore very sensitive to climate and climate change. Many of the
processes that occur in seas also occur in lakes: deltas form where rivers enter the
lake, beaches form along the margins, density currents flow down to the water bottom
and waves act on the surface. There are, however, important differences with marine
settings: the fauna and flora are distinct, the chemistry of lake waters varies from lake to
lake and certain physical processes of temperature and density stratification are unique
to lacustrine environments.

10.1 LAKES AND LACUSTRINE is retained. Lakes are typically fed by one or more
ENVIRONMENTS streams that supply water and sediment from the
surrounding hinterland. Groundwater may also feed
A lake is an inland body of water. Although some water into a lake. The amount of sediment accumu-
modern lakes may be referred to as ‘inland seas’, it is lated in lakes is small compared with marine basins,
useful to draw a distinction between water bodies that but they may be locally significant, resulting in strata
have some exchange of water with the open ocean hundreds of metres thick and covering hundreds to
(such as lagoons – 13.3.2) and those that do not, thousands of square kilometres. Sand and mud are the
which are true lakes. Lakes form where there is a most common components of lake deposits, although
depression on the land surface which is bounded by almost any other type of sediment can accumulate in
a sill such that water accumulating in the depression lacustrine (lake) environments, including limestones,