Groundwater Controls and Processes

Nash, David J.

doi:10.1002/9780470710777.ch16

Cited by 6 publications

(2 citation statements)

References 143 publications

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“…Arguments that groundwater sapping or seepage erosion played a significant role within Kalahari valley development put forward by Shaw and de Vries (1988), Nash et al (1994b) and Nash (1995) seem less likely in light of the new evidence provided by SRTM data (see section X.3.2). The valley long-profiles shown in Figures 6-8 differ from the flat or stepped longitudinal forms typical of systems generated by groundwater sapping (see Higgins, 1984;Howard et al, 1988;Nash, 2011); indeed, only four valley networks (the Auob, Passarge, Mosope and Letlhakane) exhibit predominantly flat/linear longitudinal profiles. Instead, it now appears most likely that the pronounced steps in the long profiles of valley systems such as the Mmone/Quoxo and its headwater tributaries are relict knickpoints resulting from the response of the fluvial system to regional uplift.…”

Section: X6 Theories About the Origins Of Dry Valley Systemsmentioning

confidence: 80%

Dry Valleys (Mekgacha)

Nash

2022

Landscapes and Landforms of Botswana

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Conventional rivers are absent from much of Botswana, with only the Okavango, Chobe and Zambezi systems in the extreme north containing perennial flowing water. Ephemeral rivers occur in the eastern hardveld, but the most extensive components of the surface drainage are the networks of fossil or dry valleys (termed mekgacha in Setswana and dum in various San languages) that cross the sandveld. This chapter presents the first holistic review of current knowledge about these enigmatic landforms. It does so using a range of evidence types, from radar remote-sensing to the analysis of historical documents written by missionaries and explorers. The chapter considers dry valley distribution, morphology, and contemporary and historical hydrology before discussing valley evolution over longer timescales. It concludes with a synthesis of the main arguments concerning how dry valley systems may have formed, including the balance between conventional fluvial incision and processes such as groundwater seepage erosion.

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Section: X6 Theories About the Origins Of Dry Valley Systemsmentioning

confidence: 80%

Dry Valleys (Mekgacha)

Nash

2022

Landscapes and Landforms of Botswana

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“…Concentrated erosion at the canyon headwall where the groundwater immerges would lead to undermining and collapse, followed by the breakdown and removal of material downstream by the water discharged from the groundwater spring (Figure 2). The evidence for active groundwater seepage and associated seepage weathering and erosion is compelling, and this was long considered the mechanism central to amphitheatre‐headed canyon development, such that their presence has been used to infer climatic, stratigraphic and fluvial conditions that would promote groundwater flow (Wentworth, 1928; Kochel and Piper, 1986; Howard and Kochel, 1988; Baker et al ., 1990; Nash, 2011). This interpretation, however, has been challenged recently (e.g.…”

Section: Introductionmentioning

confidence: 99%

Amphitheatre‐headed canyons of Southern Utah: Stratigraphic control of canyon morphology

Ryan

Whipple

2020

Earth Surf Processes Landf

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Amphitheatre-headed canyons are common on Earth and Mars and researchers have long sought to draw inferences about canyon-forming processes from the morphology of canyon heads and associated knickpoints, often suggesting that amphitheatre heads indicate erosion by groundwater seepage erosion. However, the conditions and processes that lead to amphitheatre-headed canyon formation have been debated for many years. We consider two hypotheses that attribute the amphitheatre-headed canyon formation to fluvial erosion of strong-over-weak stratigraphy or, alternatively, groundwater spring discharge and seepage erosion. A spatial analysis of canyon-form distribution with respect to local stratigraphy along the Escalante River and on Tarantula Mesa, Utah indicates that canyon form is most closely related to variations in local sedimentary rock strata, rather than inferred groundwater spring intensity. Lateral facies variations that affect the continuity of strong layers can induce or disrupt the formation of amphitheatres. Furthermore, we find that amphitheatre retreat rate is dictated by the interaction of fluvial processes downstream of the amphitheatre headwalls and stratigraphy, rather than waterfall and groundwater processes that likely importantly influence headwall form. We conclude that fluvial erosion of strong-over-weak stratigraphic layering alone is sufficient to form amphitheatres at knickpoints and canyon heads. Thus, we reaffirm that formation process should not be inferred from canyon-head morphology, particularly where a strong-over-weak layering is known or plausible.

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2016

The Dictionary of Physical Geography, 4th Edition

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Groundwater Controls and Processes

Cited by 6 publications

References 143 publications

Dry Valleys (Mekgacha)

Dry Valleys (Mekgacha)

Amphitheatre‐headed canyons of Southern Utah: Stratigraphic control of canyon morphology

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