Page 167 - Sedimentology and Stratigraphy

154 Lakes

10.2.1 Hydrology of freshwater lakes ture. These two divisions of the lake water are known
as the epilimnion, which is the upper, warmer lake
Lakes are relatively static bodies of water, with no water, and the hypolimnion, the lower, colder part:
currents driven by tidal processes or oceanic circu- they are separated by a surface across which the
lation (cf. seas). Waves form when a wind blows temperature changes, the thermocline (Fig. 10.3).
over the surface of the water, but the limited size of The density of pure water is determined by the tem-
any lake means that there is not a large fetch (4.4) perature and, above 48C, the density decreases as it
and hence the waves cannot grow to the sizes seen in becomes warmer. The stratification is therefore one
the world’s oceans. Wind-driven surface currents may of density as well as temperature, and, because
reach velocities up to 30 cm s 1 (Talbot & Allen the lower density warm water is above the higher
1996), especially in narrow valleys where the density cold water, the situation is stable (Talbot &
wind is funnelled by the topography. However, Allen 1996).
currents driven by the wind in lakes are too weak Agitation of the lake surface by waves and circula-
to move anything more than silt and fine sand and tion in the epilimnion means that this part of the
will not redistribute coarser sediment. These cur- water body is oxygenated by contact with the air. In
rents and the relatively small waves formed on a the hypolimnion, any oxygen is quickly used up
lake influence the upper part of the water body, and by aerobic bacterial activity and, due to the lack
the effects of the water oscillation decrease with of circulation, is not replenished. The bottom of
depth (4.4.1). Therefore, below about 10 or 20 m the lake therefore becomes anaerobic (without air,
depth the lake waters are unaffected by any wave or and therefore oxygen) and this has two important
current activity. This allows for the development consequences. First, any organic material that falls
of lake water stratification, which is seen as a through the water column to the lake floor will not
contrast in the temperature, density and the chemis- be subject to breakdown by the activity of the aerobic
try of the waters in the upper and lower parts of the processes that normally cause decomposition of plant
water body. and animal tissue. If there is abundant plant material
The surface of the lake is warmed by the Sun being swept into the lake, this has the potential to
and the water retains the heat to acquire a steady form a detrital coal layer (18.7.1), and the remains
temperature that varies gradually with the seasons. of algal or bacterial life within the lake may also
Due to the lack of circulation the water in the lower accumulate to form a bed rich in organic matter,
part of the lake remains at a constant, cooler tempera- which may ultimately form a sapropelic coal or a

Fig. 10.3 The thermal stratification of fresh lake waters results in a more oxic, upper layer, the epilimnion, and a colder,
anoxic lower layer, the hypolimnion. Sedimentation in the lake is controlled by this density stratification above and
below the thermocline.