Geology and topography
Geology refers to the structure and composition of the Earth and to the material (substrate) comprising a landscape. While seldom mentioned in the context of wetlands, knowledge of wetland geology is critical in understanding wetland dynamics.
The topography (shape of an area) and geology has a direct impact on the location and type of wetlands. These components also impact directly on other wetland characteristics (e.g. water quality, fauna, vegetation), and can be a reflection of the physical processes occurring in the wetland.
Wetland topography is one of the attributes looked at when applying the Queensland wetland habitat classification scheme.
The wetland substrate layer is the material lying below the water and may include rock, soil/sediment/clay layer (which can be derived from the underlying bedrock or deposited from other areas), peat (compacted deposit of partially decomposed organic debris, usually saturated with water) or sand. The substrate layer plays a role in determining vegetation communities and water quality. The geology of the wetland substrate also defines its porosity which determines whether water will be held at the surface or may flow through as recharge or discharge to groundwater systems.
The material underlying wetlands may be characterised into 3 broad rock categories, with variations in source, age and processes leading to formation. Sedimentary rock is generally formed by the deposition of particulate matter, metamorphic rock by the restructure of pre-existing rocks under high temperature and pressure, and igneous by the solidification of volcanic material. Geology and associated geomorphic processes—those processes that shape the landscape—operate at a variety of spatial scales to influence conditions found within a catchment, including large-scale drainage patterns and local water quality.
Drainage patterns in a catchment, including the river network and branching complexity, are reflections of the underlying geologic conditions. The geology of an area influences the types of landforms present and their spatial organisation, whilst the permeability of bedrock may determine run-off conditions and the movement of groundwater. Consequently, geology drives the way water moves through the landscape—influencing flow paths, patterns of water storage and residence times.
In addition to the rock present in a catchment, unlithified material such as sediment and soils, also influence wetlands. Catchment soil characteristics are thought to be important in determining spatial patterns of water flow paths, storage and sediments supply. For example, the turbidity of a wetland is influenced by the geology and geomorphic processes in the catchment. The type of rock present will determine the infiltration capacity and hence erosivity; whilst the geomorphic processes acting on the rock—including the erosion, transport and deposition of sediment—will lead to a particular water quality characteristic. Aquatic systems higher in the catchment are erosional, often containing coarse substrates. In comparison, those located lower in the system are depositional and generally form part of the floodplain area, containing finer substrates such as sand, silt and clay. These fine sediments may become suspended in the water column and contribute to the turbidity of a system.
Last updated: 22 March 2013
This page should be cited as:
Geology and topography, WetlandInfo, Department of Environment and Science, Queensland, viewed 14 December 2018, .