Resource Management 2 Lecture 5 (Part 2) Notes for 2024.pdf

Resource Management 2 (Lecture 5: Part 2) Hydrogeomorphic Wetland Classification  Hydrogeomorphic Wetland Classification  Six different classifications of wetlands occur across South Africa based on the underlying geology An illustrative overview showing a detailed hydrogeomorphic classifica...

Resource Management 2 (Lecture 5: Part 2) Hydrogeomorphic Wetland Classification  Hydrogeomorphic Wetland Classification  Six different classifications of wetlands occur across South Africa based on the underlying geology An illustrative overview showing a detailed hydrogeomorphic classification system to assist with wetland identification. Source: Local Action for Biodiversity: Wetland Management in a Changing Climate (2017)  Hydrogeomorphic Wetland Classification  Unless indicated to be otherwise, references are from Ollis D., Snaddon K., Job N. and Mbona N. (2013). Classification Systems for Wetlands and other Aquatic Ecosystems in South Africa. User Manual: Inland systems. SANBI Biodiversity Series, 22 Seeps  Seeps  Seeps are located on gently to steeply sloping land and dominated by the colluvial (gravity-driven), unidirectional movement of water and material downslope  Seeps are often located on the sideslopes of a valley but they do not typically extend onto a valley floor  Water inputs are primarily through subsurface flows from an upslope direction  Seeps are often associated with diffuse overland flow during and after rainfall events  Seeps  It is important to note that a seep can share a boundary with a distinct river channel and feed into the channel through diffuse surface flow or subsurface flow Seeps Seeps Depressions  Depressions  A depression wetland is an inland aquatic ecosystem with closed or near closed elevation contours  This wetland increases in depth from the perimeter to a central area of greatest depth and within which water typically accumulates  Dominant water sources are precipitation, groundwater discharge, interflow and (diffuse or concentrated) overflow  Depressions may be flat bottomed or round bottomed and may have any combination on inlets and outlets or lack them completely  Depressions  An important point to note when delineating depressions is that the extent of the wetland goes beyond the body of water which may be present within the deeper parts of the depression  Depressions which have been dammed will still often appear spherical but with one or more straight edges Depressions Depressions Wetland Flats  Wetland Flats  Wetland flats often appear as irregularly shaped wetland areas which are not linked to a stream  They are often level or near-level areas where waterlogging occurs and can be differentiated from depressions by their lack of defined margins Wetland Flats Wetland Flats Floodplain Wetlands  Floodplain Wetlands  Floodplain wetlands occur on mostly flat or gently sloping land adjacent to and formed by an alluvial river channel  These wetlands occur on a plain and are typically characterised by geomorphological features associated with river derived depositional processes, including point bars, scroll bars, oxbow lakes and levees Floodplain Wetlands Floodplain Wetlands  Floodplain Wetlands  Streams located on gentle slopes with low water velocities will flow back and forth, finding the lowest elevation to flow to  This shift in the streams course is known as meandering  Floodplain Wetlands  A meandering stream has a single channel that winds snakelike through its valley  Over time, a stream will meander back and forth across the flat valley floor, known as the floodplain  Lateral erosion, shifts the course of the stream over time  Floodplain Wetlands  As water flows around these curves, the outer edge of water is moving faster than the inner  This creates an erosional surface on the outer edge (a cut bank) and a depositional surface on the inner edge (a point bar)  Floodplain Wetlands  Meanders change position by eroding sideways and slightly downstream  Floodplain Wetlands  The sideways movement occurs because the maximum velocity of the stream shifts toward the outside of the bend, causing erosion of the outer bank  Floodplain Wetlands  At the same time the reduced current at the inside of the meander results in the deposition of coarse sediment, especially sand  Floodplain Wetlands  This lateral erosion is concentrated along the outside bank of the meandering stream (primarily due to higher water velocity)  On the inner bank (slower moving water), soil and silt is being deposited, this is known as fluvial deposition  Over time, lateral erosion and fluvial deposition gradually shift the course of the stream back and forth across the floodplain  This results in the newer stream course being steeper, generally in a more straight direction with a faster flow  Floodplain Wetlands  Where the bends of two meanders meet, they bypass the curve of river, creating an oxbow lake which may then be infilled with overwash sediment  Floodplain Wetlands  Erosion along the outer bank eats away at the soil (1)  Further erosion forms a cutoff meander and a new stream or river channel (2)  Eventually an oxbow lake is created that is separate from the stream or river (3) Floodplain Wetlands  Floodplain Wetlands  Oxbow lakes may eventually become swamps, known as oxbow swamps  If precipitation is low, these swamps may dry up and form small streams or dry stream beds known as meander scars  Floodplain Wetlands  Several types of landforms are associated with larger streams and rivers and the floodplains they produce Hills or ridges along each side of a major stream or river are known as bluffs. If river flooding occurs, it generally does not reach this level. Undercut banks or natural levees are formed from silt or soil deposits along the outer edge of the stream or river. Streams that flow parallel to the main stream are called yazoo streams. Yazoo streams are cutoff from the main stream due to natural levees. When a stream or river meanders and cuts into itself it is called a cutoff meander. This allows for the stream or river to take a shorter course. A cutoff meander will initially become an isolated lake known as an oxbow lake. When cutoff meanders fill with sediment and dry up, they become meander scars. Unchannelled Valley Bottom Wetlands  Unchannelled Valley Bottom Wetlands  Unchannelled valley bottoms will have no clear stream  This often requires the wetland assessor to closely examine the area as channels can often be narrow and difficult to find  The absence of linear riparian vegetation following a stream is a good indicator that the wetland is unchannelled Unchannelled Valley Bottom Wetlands Unchannelled Valley Bottom Wetlands Channelled Valley Bottom Wetlands  A G B F C E D  Channelled Valley Bottom Wetlands  Channelled valley bottom wetlands with visible channels are easily identified as wetlands with a stream or river passing through it  The associated wetlands will often appear as wetter, greener vegetation on one or both sides of the stream Channelled Valley Bottom Wetlands Channelled Valley Bottom Wetlands