Morphometric Parameters

In the past, mapping lakes was a laborious task. Lake shorelines had to be established by such methods as transverse, stadia or plane table surveys, all time consuming, and spot depths by lead lines at fixed positions. Today, outlines of shores are easily obtained from aerial or satellite images and depth profiling done by echosounders. Exceptions include shallow ephemeral lakes, like many in Australia, which are best mapped when dry using modern terrestrial surveying equipment. Once, the basic data had to be processed by hand to produce a map, but nowadays almost all stages can be computerized. In all cases scale is important, the finer the scale, the more accurate the map. The parameters derived from these maps can be divided into those taken directly or indirectly from the map and those that are derived by computation from the primary parameters.

(a) Primary Parameters

(i) Lake length. This is the length of the line connecting the two most remote points on the shoreline. It should not cross land (so in an oxbow lake the line is curved), but it may cross islands. Such measurements are of intrinsic geo-morphic value only. Of more use limnologically is the maximum effective length, which is defined by the longest straight line over water on which wind and waves can act. While the two values are similar in large deep lakes like Africa's Lake Victoria, in island-studded lakes like Sweden's L. Vanern, the maximum effective length is 69% of the maximum length. Knowledge of this parameter is important in geomorphic studies on shorelines and in studies on seiches and stratification in physical limnology.

(ii) Lake width is defined by a straight line at right angles to the maximum length and connecting the two most remote shorelines. Again, of more value is the maximum effective width, which does not cross land. This, and the mean width are of use mainly in hydromechanical studies.

(iii) Lake depth is the maximum known depth of a lake, and is the single most important geomorphic parameter of a lake. The world's deepest lake is Lake Baikal in Siberia (1620 m deep) and many large glacial, volcanic, and tectonic lakes exceed 200 m in depth. By contrast, most lakes on flood plains or formed by wind are shallow, rarely exceeding 5 m in depth. The limnological consequences of this are paramount and discussed later. Mean depth (lake volume/lake area) is also worth noting and has been used many times to explain varying productivities between lakes (e.g., the deeper the lake the less productive it is). Of the various other depth-related parameters, relative depth, which is the ratio of the maximum depth to the mean diameter of the lake, is useful in explaining stability of stratification in lakes. For large shallow lakes like the wind-stirred Lake Corangamite in Victoria, Australia, the value is 0.09%, while the nearby meromictic West Basin Lake in a volcanic crater, the relative depth is 3.0%.

(iv) Direction of major axis. It is important to know this in geomorphic and hydrodynamic studies as lakes may be aligned to dominant wind direction and hence be more subject to wind than others. For instance, in the eastern inland Australia, only those lakes with an axis near N-S grow spits under the influence of winds from the NW that close off the southeast corner (see later).

(v) Shoreline length is easy enough to measure by a map measurer, but is very much influenced by map scale. It is used to calculate shoreline development, a parameter used in littoral studies.

(vi) Lake area, once determined by planimetry, but now easily done with a computer, and hardly affected by map scale, is another of the most basic lake parameters. The world's largest lakes are of tectonic and of glacial erosion origin. At the individual country scale, lakes are often listed by area with an explanation for any pattern based on geomor-phic distinctiveness of the district/mode of origin. For instance in New Zealand, geology and climate explain the dominance of large piedmont glacial lakes on the South Island, and many somewhat smaller volcanic lakes on the North Island. If the area occupied by a lake fluctuates (because it is terminal or used for water supply) then it is useful to know the area at any depth and this is visualized in a hypsographic curve (Figure 1(a, c)). (vii) Lake volume is calculated from summing the volumes between each contour, though for increased accuracy different formulae are used according to lake form. This is another parameter often quoted for lakes, particularly if they are large. Most impressive in this instance is Lake Baikal's massive volume of 23 000 km3, representing one-fifth of the world's fresh water. Visualization of volume at any depth, particularly useful in lakes and reservoirs that fluctuate in depth, is achieved by a volume/depth curve (Figure 1(b, d)). In this respect, volume percentages derived from cumulative curves for reservoirs are widely quoted in the media in dry countries like Australia, where water reserves are precarious and precious. (viii) Insulosity is the percentage of the lake area occupied by islands. Though some lakes have minor islands, like subsidiary cones in crater lakes, their insulosity values are of no consequence. It is mainly in glacial ice scour lakes and other lakes with highly irregular shorelines where there are many islands that this parameter exceeds ca. 10% and assumes importance. While its worth is intrinsically geomorphic, it is sometimes equated to the value of a lake for recreation, where humans

Area (ha) 0 100 200 300 400 500 600

Volume (106 m3) 0 50 100 150 200

Area (ha) 0 100 200 300 400 500 600

10 20 3040 50 60 70

Cumulative area (%) Cumulative volume (%)

Cumulative area (%) Cumulative volume (%)

Figure 1 Absolute hypsographic (a) and volume curves (b), and relative hypsographic (c) and volume curves (d) for three maar lakes in Victoria, Australia.

require high shoreline-waterway interaction, as in Swedish lakes Vanern and Malaren. On the other hand, sailors and waterskiers require open water for their recreation activities, so small, lakes without islands are favored. Thus, there is no relationship between insulosity and recreational use of a lake!

(ix) Watershed to lake area/volume. This is easy to calculate from maps and is used as an estimate of terrestrial inputs as in eutrophica-tion studies and also as water renewal ratios for some in-lake processes. (b) Common Derived Parameters

(i) Shoreline development (Ds) is a measure of the irregularity of the shoreline; its accuracy is dependent on map scale. Essentially, it is the ratio of the length of the shoreline to the length of the circumference of a circle of area equal to that of the lake. It is an index of the potential importance of littoral influences on a lake. Perfectly circular lakes have a Ds of 1.0, average lakes have values between 1.5 and 2.5, while lakes and reservoirs with much indented shorelines have values exceeding 3 (Figure 2). Expressed in relation to lake types, volcanic vent lakes have minimal Dss, and lakes in dammed valleys and in ice-scoured terrain have the highest values.

(ii) Volume development. This index (Dv) is used to express the form of a basin, and is defined as the ratio of the volume of a lake to that of a cone of basal area equal to the area of the lake and depth equal to the maximum depth of the lake. Lakes with a Dv of 1.0 are perfectly cone-shaped and uncommon. Values <1.0 indicate a trumpted-shaped lake and are rare

Shoreline development

Shoreline development

Volume development

Figure 2 Graphical representation of shoreline development for three lakes of different shape, and of volume development for another three lakes of different volume distribution. In the latter, concentric lines represent depth contours.

Volume development

Figure 2 Graphical representation of shoreline development for three lakes of different shape, and of volume development for another three lakes of different volume distribution. In the latter, concentric lines represent depth contours.

and values >ca. 2.5 indicate a beaker-shaped lake and are also uncommon (Figure 2). Most lakes have values somewhat greater than unity. The unusual values may be related to lake type, e.g., doline lakes usually have Dvs near 1 and claypans and maar lakes may have Dvs near 3, but sometimes Dvs may be the result of peat growth/marl deposition/shoreline slumping/erosion in the littoral, leading to values near 1. In all cases the index is a surrogate for the role of the sublittoral in a lake's limnological processes, the lower the value the greater the sublittoral influence.

(iii) Slope is the angle, usually expressed as a percentage of repose of the bottom sediments and can be determined directly from an echogram or from a contour map using appropriate measurements and formulae. Slopes of near 0% (profundal areas) to 20% (sublittoral slopes) are common, but occasionally values approach the maximum of 90% in a steep-shored crater, glacial erosion and lakes due to earth movements. This parameter is used mainly in sedi-mentological studies and to lesser extent in benthic studies in choosing suitable stations. Mean slope is an average for a whole lake and is derived from a contour map and application of formulae. Not surprisingly, mean values are much more subdued than slopes at designated contours and commonly range from <1% to 10%, but may exceed 25% in some lakes, e.g., Lake Barrine, a small maar in north Queensland, Australia, has a mean slope of 30%. Perhaps in interlake comparisons, visual inspections of comparative hypsographic curves (Figure 1) are just as instructive as are figures for mean slope, and far less troublesome to prepare.

(iv) Sometimes a derived parameter can be established for a special need. For example Ratio of epilimnion sediment area to epilimnion volume has been used more effectively than parameters based on whole lake volume in eutrophication studies because it more accurately accounts for nutrient recycling in a lake.

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Responses

  • MARTIN
    What is the limnologic importance of knowing length?
    2 years ago
  • bell
    What is morphometry in aquatic system?
    9 months ago

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