Part 2

• Augrabies


  Derived from the Nama word "Akoerabis", meaning "Place of Great Thunder" or "Place of Loud Noise"
• Augrabies Falls National Park

  • Covers an area of 554 km2
  • Desert landscape formed in crystalline rocks, including igneous and metamorphic rocks
  • Features mountain ranges and various weathering landforms (bornhardts, nubbins, pediments)
• Annual rainfall average of ~125 mm in the Augrabies region
• Potential annual evapotranspiration > 2,500 mm in the Augrabies region
• Despite the large water deficit, the anabranches in the Augrabies region maintain low flows throughout the year primarily due to rainfall and snowmelt from the upper reaches of the Orange River
• The flow regime in the Augrabies region is highly variable, with larger summer floods ranging from 1,000 to 3,000 m3/s
• The bedrock geology in the Augrabies region primarily consists of granite gneiss and gneiss, with locally occurring mafic lithologies
• The Augrabies region is considered tectonically stable, but a recent earthquake swarm (2010-2012) highlights the ongoing seismic forces acting on areas of crustal weakness
• The fluvial landscape in the Augrabies region can be divided into three zones:


  1. Upstream of the Main Falls

  2. Around the Main Falls

  3. Downstream of the Main Falls
• Upstream of the Main Falls

  • Orange River Valley between Upington and Augrabies Falls (150 km)
  • World's largest mixed bedrock-alluvial anabranching channel complexes
  • ~15 anabranches that divide and rejoin, surrounding stable islands composed of either bedrock or alluvium (silt and sand)
  • Islands ~2 km wide, ~15 km long, and elevated >10 m above the normal river level
• In the Augrabies Falls region upstream of the Main Falls, alluvium is relatively scarce and the majority of the anabranches and islands are formed in granitoid bedrock
• The presence of joints, fractures, and foliations in the bedrock in the Augrabies Falls region indicates structural control and contributes to distinct characteristics of the river system
• The river in the Augrabies Falls region exhibits relatively straight sections separated by sharp bends that align with the lines of weakness in the bedrock, facilitating the continuous division of flow between anabranches and the formation of deep channels and rocky islands
• Around the Main Falls

  • Complex of anabranches terminates at different waterfalls: Main Falls in the south and Dry Falls located approximately 2.5 km to the north-west
  • Gradients of the anabranches are steeper at the waterfalls
  • Primary anabranch leading to the Main Falls drops around 30 m in elevation over a distance of 1.5 km and flows through a narrow chasm approximately 10 m deep before plunging over the lip of the falls with a drop of 50-60 m
• Downstream of the Main Falls

  • Main Falls marks the beginning of a gorge
  • Initially about 75 m deep and 100 m wide, with depth increasing downstream to a maximum of approximately 160 m and width varying between 100 and 400 m
  • Orientation of the gorge is influenced by structural control, with long, straight sections separated by abrupt right-angled bends where the gorge is widest
  • During large floods, tributaries transport sand to boulders, contributing to local mass failure and toppling of joint-bounded blocks and spalling of massive sheets, which add coarse debris to the gorge floor
  • Gorge features steep, near-vertical sidewalls and boulder bars several hundred meters wide, separated by longer, deeper pools
• Width of the gorge

  • Irregular and varies between 100 and 400 m
• Orientation of the gorge

  • Influenced by structural control
  • Long, straight sections separated by abrupt right-angled bends where the gorge is widest
• During large floods downstream of the Main Falls

  1. Tributaries transport sand to boulders

  2. Steep, near-vertical sidewalls

  3. Local mass failure occurs

  4. Toppling of joint-bounded blocks and spalling of massive sheets

  5. Contribute additional coarse debris to the gorge floor
• Boulder bars downstream of the Main Falls

  • Coarse debris, including entrained material from floods
  • Several hundred meters wide
  • Separated by longer, deeper pools
  • Not aligned with tributary junctions
  • Regularly spaced at intervals of approximately 5-7 times the gorge floor width
  • Formation likely influenced by the hydraulics of occasional high-energy floods in the gorge
  • Less common (and regularly spaced) farther downstream
• Beyond 18 km downstream of the Main Falls, the presence of a distinct gorge becomes less evident and the Orange River continues to flow in a narrow valley surrounded by deeply dissected terrain
• Waterfall and Gorge development in Augrabies Fall Region


  1. Flow concentration along dominant anabranches led to faster incision, diversion of flow, and erosion focusing

  2. Knickpoints (rapids or waterfalls) formed along dominant anabranches, resulting in gorge formation
• Principal knick points


  • Located at the head of gorges
  • Main Falls
  • Dry Falls
• Main Falls and Gorge

  Captured the majority of the flow from upstream anabranches, leading to knickpoint retreat
• Key contributors to bedrock erosion and knickpoint retreat

  • Plucking
  • Abrasion
• Gradient steepening and flow acceleration


  Caused a faster deepening rate along the primary anabranch and capture of flow from neighbouring anabranches
• Dry Falls


  • Considered a "dead channel and gorge"
  • Only active during the largest floods
• Main Falls


  Will continue to erode and retreat faster, gradually abandoning the more elevated anabranches and Dry Falls
• Initiation of knickpoints


  • Uncertain, influenced by erosion of resistant lithologies
  • Tectonic activity
  • Continental-scale uplift
• Main Falls could represent a knickpoint that retreated over > 600 km inland over Ma
• Knickpoint retreat and gorge formation represent a renewed stage of landscape denudation in an initially low-relief valley floor
• Gorge deepening


  Can initiate base-level falls for tributaries (anabranches), leading to knickpoint initiation and retreat along their courses
• Evidence of long-distance knickpoint retreat observed west of the Augrabies Falls region, where the Orange River flows in a deep valley without a distinct gorge, and tributaries show evidence of hanging valleys and dry waterfalls, indicating short-distance knickpoint retreat
• Despite local tributary incision and mass failure, the main gorge sidewalls remain steep and the anabranching complex on the valley floor surrounding the gorge remains intact
• Drop in baselevel


  Will drive renewed landscape denudation, leading to the dominance of the primary anabranch and reversion to a single channel cascading into a deep gorge
• Waterfall retreat rates


  • Sedimentary rocks - centimetres to a few meters per year
  • Resistant igneous or metamorphic rocks - a few millimeters per year
• Main Falls retreat rate

  Long-term average of 1 mm/year
• Minimum length of the downstream gorge (18 km) would have formed over at least 18 million years, or a retreat rate of 10 mm/year would indicate a gorge formation over at least 1.8 million years
• Retreat rates are imperceptible on a human timescale
• Cosmogenic nuclide data indicate that local denudation rates are faster along anabranches upstream of the falls compared to the surrounding interfluves and the gorge below the Main Falls, suggesting that local relief is slowly increasing over time near the major knickpoint, supported by evidence of plucking, potholes, and other abrasional features along the primary anabranch
• Significance of the Fluvial Landscape of the Augrabies

  • Orange River is crucial in shaping the geomorphological development of southern Africa
  • Bedrock incision and knickpoint retreat influence landscape denudation patterns
  • The Augrabies Falls region serves as a prime example of gorge formation and landscape denudation associated with retreating knickpoints
  • Main Falls retreat is lengthening the gorge, initiating renewed landscape denudation in the area