Seminar Sum-Up: Hutton Club 19/10/18


Rivers are at the forefront of research today – with climate change set to increase temperatures and in response alter precipitation patters estimating the consequence on river catchments is vitally important. Many aspects of rivers are still relatively unconstrained especially sediment patterns, geomorphology and the influence of tectonics. Hugh Sinclair focused in this seminar on the Himalayas – an area of great research interest which is tectonically active and a home for many of the world’s major rivers and 52.7 million people.

 Generally, it is accepted river catchments have a regularity and in mountain areas have a 2:1 geometry, so the average spacing of drainage outlets relative to the width of the mountain flank is highly uniform, seen in Figure 1. However, especially in areas like the Himalayas, seen in Figure 2, this regularity is affected by tectonics as discharge and gradients of the river change in response to faulting. The shape and distribution of the river catchment plays an important role in sediment dispersal and the Himalayas had an anomalously high sediment flux due to the mountains and this is particularly important when looking at flood risk.




 Fig 1: Spacing of drainage outlet relative to the width of the mountain flank is 2:1 in these equally spaced streams (Adapted from Hovius,1996).

 Catchments in the lee of the Himalaya (N)


 Fig 2: Location of the Himalayan mountain front and subsequent river catchments.






Hugh and his team first tackled studying the river catchments to the North of the Himalayan mountain front. Using Optically stimulated luminescence (OSL) on terraces in this area the age and subsequently erosion rate was calculated and then compared to the front of the Himalaya. OSL is a technique used to date the last time Quartz in the sediment was exposed to light giving indication of erosion rates as it records when the quartz was buried by sediment and shut off from sunlight.

 Catchments in the front of the Himalayas (S)

 At the southern flank of the mountain front river catchments seem to follow an equant pattern and unlike the usual elongate 2:1 form, they have a 1 or <1 aspect ratio. Previously it was thought tectonics and deformation had ‘squished’ the catchments to give this appearance however this hypothesis came with many unresolved issues leaving room for new research and the main focus of Hugh’s seminar.
With new techniques to show river mobility it became clear how mobile these catchments are and their tendency to migrate laterally – when they migrate catchments can capture drainage of the catchments either side and this lowers the erosional and transport capacity of the river network leading to sediment accumulation – drainage divides then move up and down in response to the changing of erosional capacity of river on either side.
At this area the largest catchments have the lowest aspect ratio so it is possible this is a function of the drainage catching process.
Focusing on the Kathmandu catchment – here there is active deformation, huge accumulation of sediment over 2.8Myr and evidence that it lies perched, a whole km higher than base level on this gigantic plain. So, it is likely the Kanali catchment, laterally next door, has migrated into this valley, capturing the Kathmandu catchment forcing a change in the drainage divide and so the aspect ratio.

Conclusion

 The catchment starts as a 2:1 system but perching of the river and aggradation causes catchments to be victims of divide migration and thus alteration of the catchment aspect ratio. This aggradation can be linked to variations in base level along strike as some rivers in the south drain into the ganga plain and some into a perched base level that is due to the fold and thrust belt here, particularly a propagation thrust which creates a wedge top basin.

Implications

 The smaller catchments have very peaked rainfall and receive a lot of precipitation as they lie at the mountain front and so cause localised flooding. However, larger catchments such as the Kosi have a huge influence, these rivers flow into plains, with heavy infrequent rainfall sediment is mobilised and can cause switching of river channels – which is highly dangerous for the populations living in the delta. Research continues in this field, but the interactions of the catchments and as result sediment will be vital in the future to quantifying and understand flooding dynamics in the Himalayas.

- Meg


Hovius, N. (1996). Regular spacing of drainage outlets from linear mountain belts. Basin Research, 8(1), pp.29-44.

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