Alluvia—lower catchment

Alluvia—lower catchment

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Alluvial aquifers are formed from particles such as sand, silt and/or clay deposited by physical processes in river channels or on floodplains. Alluvia can contain one or more unconfined, unconsolidated sedimentary aquifers, where groundwater is stored and transmitted through intergranular voids between gravel and sand particles. These unconsolidated sedimentary aquifers may be layered and/or discontinuous due to the presence of deposits of low permeability silt and clay within the alluvia. Alluvia in lower catchment areas tend to be significantly wider and deeper than alluvia further up-catchment. Alluvia may also contain a number of paleo-channels, remnants of old channels and riverbeds. Paleo-channels may transmit groundwater faster than the surrounding alluvia and may also form perched aquifers under certain conditions. Underlying the alluvia may be an impermeable rock layer which would act as a confining layer separating the unconfined sedimentary aquifer in the alluvia from other groundwater bearing geologies. Flood events provide significant recharge of alluvial aquifers (see Recharge conceptual model).

Unconsolidated sedimentary aquifers in lower catchment alluvial deposits may provide a range of ecosystems with water required to support their plant and animal communities, ecological processes and delivery of ecosystem services.

• Palustrine (e.g. swamps) and lacustrine (e.g. lakes) wetlands and riverine (e.g. streams and rivers) water bodies on alluvial deposits may depend on the surface expression of groundwater from these unconsolidated sedimentary aquifers.
• Terrestrial vegetation located on alluvial deposits may depend on the subsurface presence of groundwater in these unconsolidated sedimentary aquifers where groundwater is typically accessed through the capillary zone above the water table.
• Unconsolidated sedimentary aquifers in alluvial deposits may also support ecosystems within the aquifer itself, which sometimes is indicated by the presence of stygofauna.

Pictorial conceptual model PDF

Last updated: 18 December 2015