Methods Manual for Salt Lake Studies/Describing your lake - hydrogeomorphology

Authors: PSJ Coleman

Overview

Salt lakes share some common hydrologic, soil, and vegetative characteristics. Beyond these general similarities, however, there is a very wide variation in terms of their size, complexity, physical, chemical, and biological characteristics and functional processes. Attempts to develop a simple classification for lakes abound in the literature (Timms, 1992). Most definitions start with the geological origin of the lake then move on through the geomorphology of the lake basin to the ionic composition of the waters, permanance of the waters, depth of the waters etc.

Hydrogeomorphic classification

The hydrogeomorphic classification approach developed by Brinson (1993) to identify groups of wetlands that function similarly has relevance when applied to salt lakes. The hydrogeomorphic classification is based on three fundamental factors that influence how wetlands function:

1. geomorphic setting (landform elements, geologic evolution, position in landscape),
2. water source (rain, runoff, groundwater), and
3. hydrodynamics (energy level of any moving water, direction of flow)

Examples of how these three factors influence the wetland types can be seen in the examples shown in the table below.

Hydrogeomorphic summaries of some types of Australian wetlands (based on Smith et al, 1995)

The objective of the characterisation stage of a salt lake study is to document existing conditions, thus providing a reference point for comparing future conditions and a better understanding of the processes occurring in the lake. To meet this objective a description of the climate (including BoM climate averages where available), landform and geomorphic setting, apparent hydrology, vegetation, soils, groundwater features, surficial geology, and other relevant characteristics can be used to develop a hydrogeomorphic classification the lake.

Mapping

It is also useful to develop a base map of your lake and its surrounding landscape. Multiple maps or overlays, keyed to the base map, may be used to show features of interest. The map should display the following information:

1. Topographic contour lines in the project area and surrounding landscape,
2. Infrastructure (e.g., roads, fences, buildings, railroad grades, and bridges),
3. Surface water features (e.g., streams, rivers, lakes, ponds, and springs),
4. North arrow (true north), legend or key, and distance scale, and
5. Title block with the project name, investigators, dates, and information sources.

Overlays may be developed from work conducted in other phases of the project, and these may include:

1. Soil types,
2. Plant communities, and
3. Location of potential impacting asapects.

The water regime

The period salt lakes remain inundated varies from a few days at a time to permanent. The pattern of wetting and drying is called the hydropoeriod and is an important driver of processes within the lake.

To determine the seasonal variations in hydrology, a series of depth posts may be placed perpendicular to the lake's edge, at several locations around the lake. This will allow the recording of water depth (from an arbitrary datum point) and extent of inundation over the seasons. The height of water on these pegs should be recorded at regular intervals.

Another method of assessing regular changes in inundation patterns is to review historic aerial photography.

During site visits in dry season, measuring the depth to free water and depth to saturated soil in a pit is informative.

The chemistry (composition and concentration) of the overlying brines impacts on the evaporation rate from the surface and may therefore influence on the hydroperiod. Effects of salinity on evaporation rate

Salts dissolved in the brine become saturated at different stages, allowing sequential crystallisation of brine salts as a salt lake dries up. [[1]]

Soils

Soil description methods and soil taxonomies vary from country to country. Some resources to assist in assessing your lake's soils follow:

1. Isbell RF (1996) The Australian Soil Classification, CSIRO Publishing, Collingwood Victoria
2. Mc Donald RC, RF Isbell, JC SPeight, J Walker and MS Hopkins (1990) Australian Soil and Land Survey - Field Handbook 2^nd ed, CSIRO Publishing, Collingwood Victoria
3. Soil Survey Staff (1951) "Soil Survey Manual" USDA Agricultural Handbook No 18, Gov't Printer, Washington DC
4. Soil Survey Staff (1996) Keys to Soil Taxonomy, 7^th ed, Pocahontas Press, Blacksburg, Virginia