Wetland Flat

—a level or near-level wetland area that is not fed by water from a river channel, and which is typically situated on a plain or a bench. Closed elevation contours are not evident around the edge of a wetland flat.

This HGM Type was added to the Classification System in order to provide a way of classifying wetland areas on flat land (often on a coastal plain) that are not in any way connected to the drainage network (i.e. there are no river channels flowing into, out of, or through the system), and for which none of the other HGM Types seem to be appropriate.

The primary source of water for a wetland flat is precipitation, with the exception of wetland flats situated on a coastal plain where groundwater may rise to or near the ground surface. Dominant hydrodynamics in wetland flats are bidirectional vertical fluctuations, with weakly developed multidirectional horizontal water flows present in some cases. Water typically exits a wetland flat through evapotranspiration and infiltration.
Wetland flats are characterised by the dominance of vertical water movements associated with precipitation, groundwater inflow, infiltration and evapotranspiration. Horizontal water movements within these wetlands, if present, are multi-directional, due to the lack of any significant change in gradient within the wetland.

It is important not to confuse Wetlands Flats with Floodplain Flats.

Seep

—a wetland area located on gently to steeply sloping land and dominated by colluvial (i.e. gravity-driven), unidirectional movement of water and material down-slope.

Seeps are often located on the side-slopes of a valley but they do not, typically, extend onto a valley floor.
Water inputs are primarily via subsurface flows from an up-slope direction. Water movement through the seep is mainly in the form of interflow, with diffuse overland flow (known as sheetwash) often being significant during and after rainfall events.

Seeps are characterised by their association with geological formations (lithologies) and topographic positions that either cause groundwater to discharge to the land surface or rain-derived water to ‘seep’ down-slope as subsurface interflow. Examples of places where these conditions occur are
(1) on slopes where the water table intersects the land surface, resulting in groundwater discharge directly to the land surface;
(2) land that is down-slope of a break in slope of the groundwater table;
(3) where subsurface discontinuities in geological units (e.g. faults) cause upward movement of roundwater; or
(4) on slopes where a relatively impervious subsoil layer impedes the infiltration of rainderived water into the ground.

It is important to bear in mind that seeps can occur in relatively flat or very gently-sloping landscapes, as long as there is sufficient slope for there to be a unidirectional subsurface flow of water.

As for depressions, you can further classify seeps according to their outflow drainage characteristics at Level 4B.

Depression

—a wetland or aquatic ecosystem with closed (or near-closed) elevation contours, which increases in depth from the perimeter to a central area of greatest depth and within which water typically accumulates.

Although they may at times have a river flowing into or out of them, depressions are especially characterised by their closed (or at least near-closed) contour shape, which makes them relatively easy to identify on topographic maps.
Depressions may be flat-bottomed (in which case they are often referred to as pans) or round-bottomed and may have any combination of inlets and outlets or lack them completely. Natural depressions may have no inlets or outlets (i.e. Drakensberg tarn and Sirkelsvlei), or an outlet channel (i.e. Bass Lake), or may have an inlet channel but no outlet channels (i.e. Burgerspan).
Some depressions are so extensive that, at first glance, they may appear to be large flat areas that could be classified as ‘wetland flats’ or ‘floodplain flats’, whereas over a long distance they are actually pan-shaped or basin-shaped features, meaning that they are actually ‘depressions’. An extreme example of this is the famous Makgadikgadi Pans in Botswana. To confirm whether an apparently flat area has a depressional shape over a wide distance, you should consult a topographical map with contour lines.

Most depressions occur either where the water table intercepts the land surface (such as on coastal plains along the South African coastline), or in semi-arid settings where a lack of sufficient water inputs prevents areas where water accumulates from forming a connection with the open drainage network. Lakes are a type of depression that typically forms in a valley floor, where some sort of obstruction leads to the ‘drowning’ of the valley through the accumulation of water behind a barrier (in the case of a dam, which can be considered an artificial lake, the barrier has been created by human intervention).
The dominant water inputs and outputs of a depression are dictated primarily by the outflow and inflow drainage characteristics, as classified at Levels 4B and 4C, respectively. The hydrodynamics of a depression are, however, typically dominated by vertical water level fluctuations.

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.

Channelled Valley-bottom Wetland

—a valley-bottom wetland with a river channel running through it.

Channelled valley-bottom wetlands must be considered as wetland ecosystems that are distinct from, but sometimes associated with, the adjacent river channel itself, which must be classified as a ‘river’. Remember that some river channels, especially in the more arid parts of South Africa, are vegetated.

Channelled valley-bottom wetlands are characterised by their location on valley floors, the absence of characteristic floodplain features and the presence of a river channel flowing through the wetland.

Dominant water inputs to these wetlands are from the river channel flowing through the wetland, either as surface flow resulting from flooding or as sub surface flow, and/or from adjacent valley-side slopes (as overland flow or interflow). Water generally moves through the wetland as diffuse surface flow, although occasional, short-lived concentrated flows are possible during flooding events. Water generally exits a channelled valley-bottom wetland in the form of diffuse surface or subsurface flow into the adjacent river, with infiltration into the ground and evapotranspiration of water from these wetlands also being potentially significant.

Floodplain Wetland

—a wetland area on the mostly flat or gently-sloping land adjacent to and formed by an alluvial river channel, under its present climate and sediment load, which is subject to periodic inundation by overtopping of the channel bank.

Floodplain wetlands, as the name implies, generally occur on a plain and are typically characterised by a suite of geomorphological features associated with river-derived depositional processes, including point bars, scroll bars, oxbow lakes and levees

Floodplain wetlands must be considered as wetland ecosystems that are distinct from but associated with the adjacent river channel itself, which must be classified as a ‘river’. Remember that some river channels, especially in the more arid parts of South Africa, are vegetated.
The definition of a floodplain wetland contains several key points that you should take note of. Firstly, a floodplain is a depositional surface formed by an alluvial river, not an erosional surface or a surface formed by other non-riverine processes that can deposit sediments (such as winddriven sands). Secondly, the floodplain is formed under the current climate and sediment load. Flat surfaces may be present along the margins of a river from previous eras of differing climate and/or sediment load, and these surfaces are called terraces. Terraces are generally not geomorphologically active, that is, they are not currently being built by river depositional processes. Finally, the floodplain is flooded (or inundated), on average, several times per year, during moderate peak flow events (such as a 1.5-year or 2-year flood). Terraces may be overtopped, but only by larger, less frequent floods (e.g. 50-year or 100-year events). The inner edge of the floodplain is called bankfull. The bankfull channel, or active channel, refers to the channel cross-section below the elevation of the floodplain.
Regular (or significant historical) water and sediment contributions from an associated river channel are what characterise the dynamic nature of a floodplain wetland. Another key characteristic of most floodplain wetlands is that they are generally located on a plain in terms of their landscape setting, although they can occur along the floor of a relatively wide valley with a low gradient (e.g. alongside the lower reaches of a Lower Foothill River).

Water and sediment enter floodplain wetlands mainly as overspill from a major river channel during flooding. Water movement through the wetland is predominantly horizontal and bidirectional (i.e. in and out of the wetland), in the form of diffuse surface or subsurface flow, although significant temporary containment of water may occur in floodplain depressions. Water generally exits a floodplain wetland as diffuse surface and/or subsurface flow (often returning to the river channel), but infiltration and evapotranspiration of water from a floodplain wetland can also be significant, particularly if there are a number of depressional areas within the wetland.

River

—a linear landform with clearly discernable bed and banks, which permanently or periodically carries a concentrated flow of water.
A river is taken to include both the active channel and the riparian zone as a unit.

Dominant water sources for rivers include concentrated surface flow from upstream channels and tributaries. Other inputs can include diffuse surface or subsurface flow (e.g. from an upstream seepage wetland), interflow (e.g. from valley side-slopes), and/or groundwater inflow (e.g. via springs). Water moves through the system, at least periodically, as concentrated flow and usually exits as such, except
where there is a sudden decrease in gradient causing the outflow to become diffuse (in which case the river would grade into one of the wetland types). Other water outputs from a river include evapotranspiration and infiltration.

Concentrated, unidirectional flow within a distinct active channel, either permanently or periodically, is what characterises a river.
As such, one of the key features to look for when trying to determine whether a particular Inland System is a river or not is the presence of relatively obvious channel banks (which may not be easy to determine visually in the case of well-vegetated systems) and/or a concentrated flow of water within a distinct channel assuming the river is flowing at the time of your site visit)

Key 2: HGM Unit (Level 4)

1. Is the Inland System of a linear landform with clearly discernable bed and banks, which permanently or periodically carries a concentrated flow of water?
   YES .......................................................... River
   NO . . . goto 2

2. Is the Inland System a wetland (or, in some cases, an open waterbody), which is situated adjacent or close to a distinct active channel of a river and likely to be subject to water input from periodic (intermittent to seasonal) overtopping of the channel banks?
   YES . . . goto 3
   NO . . . goto 4

3. Is the wetland (or open waterbody) located on a valley floor and likely to receive water via (surface and subsurface) runoff from one or both of the adjacent valley side-slopes, with an absence of river-derived depositional features that are characteristic of a floodplain (such as levees)?
   YES .......................................................... Channelled Valley-bottom Wetland
   NO .......................................................... Floodplain Wetland

4. Is the Inland System a wetland (typically vegetated) without clearly discernable channel banks, which is characterised by a permanent or periodic, diffuse, unidirectional through-flow of water (often dominated by subsurface flow)?
   YES . . . goto 5
   NO . . . goto 6

5. Is the wetland located on a valley floor?
   YES .......................................................... Unchannelled Valley-bottom Wetland
   NO .......................................................... Seep

6. Is the Inland System a wetland or open waterbody with closed (or near-closed) elevation contours that increases in depth from the perimeter to a central area of greatest depth, and within which water typically accumulates?
   YES .......................................................... Depression
   NO . . . goto 7

7. Is the Inland System a level or near-level wetland that is not fed by water from a river channel, typically located on a plain or a bench, and which is dominated by vertical water movements (horizontal water movements are very weak and multi-directional, if present at all)?
   YES .......................................................... Wetland flat
   NO . . . go back to 1