Unchannelled Valley-bottom Wetland

Unchannelled Valley-bottom Wetland

—a valley-bottom wetland without a river channel running through it.

Unchannelled valley-bottom wetlands are characterised by their location on valley floors, an absence of distinct channel banks, and the prevalence of diffuse flows. These wetlands are generally formed when a river channel loses confinement and spreads out over a wider area, causing the concentrated flow associated with the river channel to change to diffuse flow (i.e. the river becomes an unchannelled valley-bottom wetland). This is typically due to a change in gradient brought about by a change in base level at the downstream edge of the wetland (for example, where an erosion-resistant dolerite dyke is present) and the resulting accumulation of sediment. In some cases, an unchannelled valley-bottom wetland could occur at the downstream end of a seep, where a slope grades into a valley near the head of a drainage line. This is typical of highlands such as the Drakensberg Mountains.

Water inputs are typically from an upstream channel that becomes dominated by diffuse (surface and subsurface) flow as it enters the wetland and seepage from adjacent slopes. There may also be groundwater input into the wetland. Water characteristically moves through the wetland in the form of diffuse surface or subsurface flow, but the outflow may be in the form of either diffuse or concentrated surface flow. Infiltration and evapotranspiration from unchannelled valley-bottom wetlands can be significant, but horizontal, unidirectional, diffuse surface flow tends to dominate these wetland systems.

Remember that some river channels are vegetated, especially in the more arid parts of South Africa where non-perennial or weakly perennial flow regimes are common. These river channels may, at first sight, appear to be unchannelled valley-bottom wetlands. The key features to look for when trying to distinguish between these two HGM Types are channel banks and evidence of periodic, concentrated water flow within a channel, both of which would be present in the case of a river.

Posted on February 8, 2023 09:06 PM by tonyrebelo