Parameters of the Geomorphic Lake Types

Structural (Tectonic) Lakes

As shown above, many of these like Lakes Baikal and Tanganyika, are especially large, deep and old, so that they have special limnological features, that are expressed most meaningfully by their Touchart coefficients. Such lakes have Dvs a little above unity (1.2-1.5) and moderate Ds (1.3-3.4). Many older tectonic lakes of moderate depth (e.g., Caspain Sea, Aral Sea) have Dv s <1, while the shallow intermittent ones vary according to hydrological condition. For instance in Lake Eyre Dv decreases from 1.68 when 'full'(=5.7 m deep) to 1.55 at 4 m deep to 1.43 at 1 m deep, while Ds decreases from 4.7 when full (it spreads up entering creeks to give a highly indented shoreline) to much lower values when contained well within the vast salt flats. The limnology of Lake Eyre is different when full to when near empty, largely because of differing water salinities but the consequences of its differing geomorphic parameters contribute too. Lake Chad also has differing geomorphologies at different water levels, and probably so do many other shallow arid zone lakes. In addition, tectonic lakes in terminal basins have many stranded beach features useful in deducting past histories, as in many Tibetan lakes to give one set of numerous possible examples.

Meteoritic Impact Lakes

These are rare, but impressive because of their roundness (Ds = 1). Depths vary, but Lake Ungava (=Chubb) in Quebec Province, Canada, is 251m deep and diameter about 3 km. Intermittent Lake Acraman in South Australia, although not quite round is notable because of its huge size (diameter 22 km).

Volcanic Lakes

Like structural lakes, there is no one set of parameters which characterize all volcanic lakes, though some subtypes have characteristic features. One group are the maars and calderas; generally these have low Dss (near 1), high Dvs (near 2), high littoral slopes and low profundal slopes. In an analysis of 15 Australian maars, Ds averaged 1.14 (lowest 1.01) and Dv 1.91 (highest 2.8) (Figure 4). Such lakes are

Lake Bathymetric Chart Australia
Figure 4 Bathymetric map of two maar lakes in western Victoria, Australia. Lake Purrumbete lies almost wholly within its crater, while Lake Bullenmerri lies within three adjoining craters. Both have a Ds a little >1 and a Dv near 2.

typically dominated by their limnetic zone and have physicochemical processes not too dissimilar to the classic large lakes of limnological literature. On the other hand shallower lakes on Victorian lava fields are not that different (except for their saline waters) to shallower lakes elsewhere. These are dominated by shallow water and littoral processes due to low maximum and mean depths, low littoral slopes and often high shoreline developments. Again, the basic geo-morphic differences between the various types of lakes are expressed in depth and size, and perhaps there is scope in the Touchart system to differentiate lakes with coefficients <1 within his third type.

Glacial Lakes

Again there is a wide variety of limnologies better explained by the size-age coefficient of Touchart (group 2, see above) than by geomorphic parameters alone. Nevertheless for the piedmont lakes due to glacial erosion, waters are deep, and Dvs are high. For 10 such large lakes on the South Island of New Zealand, 7 are deeper than 200 m and the average Dv is 1.75. Shore and littoral features are unimportant compared with the contribution of large and deep limnetic region. Cirque lakes for another group, but their geomorphic parameters are hardly unique and influencial. The smaller kettles and the like are dominated by their small overall size and hence littoral processes.

Fluviatile Lakes

Almost all of these are small, and even if >10 km2 are still shallow and hence belong to Touchart's group 3 and so littoral dominated. Two common types have some characteristic parameters: most oxbows are lunate-shaped with a curving maximum length and moderate Dss (<2.5), while most blocked valley lakes are dendritic with higher Dss (2-4). Some, e.g., the bog lakes of the Waikato, New Zealand have shorelines much smoothed by growth of littoral vegetation so that Dss are low, as are Dvs.

Solution Lakes

As for fluviatile lakes most of these, especially dolines, are small and shallow, and though uvalas and poljes may be larger in area, they are still shallow. Perhaps the parameter of most interest is their Dv -this is often 1 or less, indicating a cone or trumpet-shaped basin. Unlike the previous fluviatile lakes, but like kettles, they may have multiple deeper subbasins.

Aeolian Lakes

Lakes formed, or largely moulded by wind action, are generally shallow (<5 m, often <1 m deep), relatively small (<100 km2, often <1km2) and have sandy shores, so that shore processes are most important in their limnology. Hence spits, beaches, islands, and deltas feature in their evolution and their basin orientation is both a consequence of their development and/or a causal driver in their further evolution. Orientation to wind and effective maximum lengths for wave generation are basic controllers of their geomorphology. Many lakes in southwestern Western Australia and elsewhere are rounded or ellipitical and difference in orientation and size between areas is related to different wind directions and/or rainfall distribution. On the other side of Australia, only those lakes orientated with their major axis N-S, develop spits in the southeastern corner in an attempt at lake compartmentalization (see later) (Figure 5). These spits increase Ds, which increases the habitat for shorebirds. Under unidirectional or bidirectional winds, shallow lakes may completely segment (i.e., form separate compartments), creating smaller lakes with more shorelines and increasing the important of littoral processes.

Inflow

Inflow

Spit

Flood outflow

Figure 5 Bathymetric map of Lake Yumberarra, Outback Queensland, Australia. Key: beach ridges long dashes; creek channels, short dashes. Depth contours inverted from normal pattern, as this is more convenient in shallow intermittent lakes with highly variable water levels.

s. Inflow

Spit

Flood outflow

Figure 5 Bathymetric map of Lake Yumberarra, Outback Queensland, Australia. Key: beach ridges long dashes; creek channels, short dashes. Depth contours inverted from normal pattern, as this is more convenient in shallow intermittent lakes with highly variable water levels.

Coastal Lakes

Although most coastal lakes are due to sea-level rise, their geomorphology is variable due to different inherited features. A few are in fjords, so may be deep and steep-sided, while most are in drowned valleys, so dendritic or triangular, and those on low coasts may be elliptical. Australia's southeast coast has numerous coastal lakes of many subtypes, as does the Landes coast of France, the Cape Cod coast of North America, the Baltic coast to name a few others. Their active depositional environment encourages delta and spit formation, and lake compartmentilization in low coasts, so that shorelines are longer than those inherited. Given most lie in windy environs, basin orientation, and effective lengths are particularly important for those lake processes dependent on wave generation.

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