Geology and Topography

Geology refers to the soil structure and underlying material. Geology may differ significantly between upland and lowland parts of a province. Upland channels are erosional, and often contain coarse substrates, whereas lowland channels are depositional, and generally part of a much larger floodplain area, containing finer substrates such as sand, silt and clay.  Fine particles, such as silt and clay, can also become suspended in the water column and contribute to turbidity of a system. 

Underlying material can originate from a number of sources, and be of differing ages. Sedimentary rock is generally formed by the deposition of particulate matter, metamorphic rock is formed by the restructure of pre-existing rocks by high temperature and pressure, and igneous rock is formed by the solidification of molten magma.

Cracking clays allow organic matter into the soil, eventually providing nutrients to terrestrial vegetation. Prior to cracking, nutrient input to vegetation may be limited. Cracking also allows access and movement of animals throughout the soil, and may provide refugia for some biota. Deep soil penetration during flooding can be achieved if the surface soil has had sufficient time to dry and crack (Young, 2001). This groundwater is available to floodplain trees, and may be very important when surface conditions are dry.

Geology determines the types of groundwater aquifers that exist, their extent, and how easily they can be utilised by vegetation. Underlying geology may also determine the extent to which a river is supplied by groundwater. Surface soil properties and underlying geology dictate the vegetation communities that are present.

Underlying geology can influence some aspects of water quality. For example, limestone is alkaline, and large areas of underlying limestone may result in an alkaline river or stream. Acidity and alkalinity have major influences on natural wildlife distributions (Jeffries and Mills, 1990), due to species-specific tolerance ranges of varying pH. Some taxa are capable of tolerating a wide pH range, whereas others are not. Differing geologies throughout a catchment may limit the occurrence of some species to a particular area where the pH is at a level that they can tolerate.

Fluctuations in temperature and intermittent flooding can cause the soil surface to heave upwards (Young, 2001), resulting in the formation of depressions and small rises. Ponds, of rain and floodwater, form in these small depressions for a longer period of time than on the surrounding floodplain, and may result in a different vegetation community.  For example, Lignum (Muehlenbeckia florulenta) grows on cracking clay soils that typically have uneven surfaces (Young, 2001). The presence of such micro-habitats may result in a higher biodiversity for a region than its surroundings. 

Primary and secondary production in tropical stream systems is known to be influenced by catchment lithography, with basalts considered to be nutrient-rich, and granites nutrient-poor (Pringle et al, 1990). Streams that drain granitic or largely siliceous sandy catchments with nutrient-poor soils may derive most of their nutrient inputs from the riparian zone (Pusey and Arthington, 2003).