Changes with Time

Over geological time, lakes are temporary landscape features (but see later), filling with sediment and/or eroding at the outlet. Consequently lake morphometry changes with time, imperceptibly in well-watered areas, but at the other extreme, oscillating widely in terminal lakes in arid areas. The most obvious changes are lake level fluctuations and their concomittent shoreline changes. Sedimentation is less obvious, except for delta construction.

Lowered lake levels, of whatever cause, result in stranded beaches, spits and deltas. Worldwide, piedmont glacial lakes provide many examples, perhaps none better than New Zealand's Lake Wakatipu with its 10 strandlines cut into a major bench and deltaic fronts at 50 m above present lake level The terminal lakes of Tibet have numerous elevated shorelines attesting to a more fluvial past, as do most of the world's large lakes in endorheic regions. Interesting as these landforms may be, and useful in aging a lake, it is the changes with decreasing depth in physicochemi-cal processes in the lake such as thermal and chemical stratification, that are important limnologically.

In some lakes, levels have been elevated since initial formation, drowning shoreline features and river channels. Coastal marine lagoons, tied to changing sea levels, provide many examples including Lake Mac-quarie near Sydney, Australia, where drowned river channels and spits are clearly discernable (Figure 3). As in lakes with lowered levels, raised levels may change physicochemical processes, and in addition, there may be hydrodynamic changes associated with changed inflows.

All lakes accumulate sediment, either mainly from stream inflows, or biological production in the lake itself, or occasionally from overland flows and rarely by showering ash from volcanic eruptions (as in lakes around Taupo and Rotorua in New Zealand). Rates

Figure 3 The western bay of Lake Macquarie, near Sydney, Australia, a drowned coastal valley, showing the former river course and a lobate delta.

are highly variable and depend on geomorphic type, catchment size and erodability, and trophic status (and proximity to a volcano!). The most rapid accumulation is by delta building in lakes with large inflows in mountainous areas. Well-known examples include the deltaic plain at the head of Lake Geneva and the surficial sediments that divide Thuner See and Brienzer See at Interlaken, both in Switzerland. Deltaic form depends on the relative densities of lake and river water and lake currents. Arcuate Gilbert deltas due to homopycnal flows are perhaps the most common type in piedmont glacial lakes, lobate deltas associated with hypopycnal inflows in coastal marine lagoons (Figure 3), and alluvial fans in shallow desert lakes. No matter the deltaic form, lake surface area and volume are reduced, but probably with minimal influence on limnological processes in large lakes.

On the other hand, lake floor sedimentation due to submarine delta building by hyperpycnal inflows (as in Lake Pukaki in New Zealand) and by chemical or biological deposition decrease lake depth and hence may influence physicochemical processes, particularly if the sediments of productive lakes consume oxygen during stratification. Rearrangement of bottom sediments due to slumping or bioturbidation are of little geomorphic consequence, but may upset layering and hence interpretations of lake/catchment history recorded in sediment cores.

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