For all you geologically inclined...

This is a project I worked on for our trip.

Landscape Degradation in the Himalaya

Christine Rahtz

UC Geology 2011-New Delhi to Leh

Table of Contents

Introduction………………………………p. 2

Anthropogenic Influence…………………..p. 3

Precipitation and Erosion………………….p. 5

Mass Movement………………………….p. 8

Future Research…………………………..p. 10

Introduction

There is no doubt that erosion processes of the Himalaya are quite complex, and affected by several components, including tectonics, anthropogenic land use and vegetation, and glacial and fluvial processes. The relationship between climate, tectonics and rates of erosion in the Himalaya is one of extensive scientific exploration.  The connection between erosion rates and precipitation in a region of tectonic uplift is widely debated and not well known.  Some hypothesize that the linkage is characterized by a cycle of uplift, followed by increased orographic precipitation, higher rates of erosion, unloading of the crust, and further uplift.  There are many causes, natural and unnatural, of landscape degradation and soil erosion in this environment. This webpage explores and explains the complexities of the erosional landscape of the Himalayas. 

Anthropogenic Influence

“The construction of highways may actually accelerate some of the geomorphological processes in the region,” writes Lewis Owen, a geomorphologist at University of Cincinnati (1996).  Road construction in the Himalaya can create many problems, including increased erosion and landslides.  Road construction is important for local people, it allows them mobility and access to resources.  It is however, extremely dangerous, and thousands die in the building of roads through these mountains (Owen 1996).  These roads do not last very long either.  In my travels through India, I learned that patience is an absolute must.  There were several times during our two week trip that we had to sit and wait for a slope failure to be cleared from the road.   

Deforestation is another human induced activity that can lead to landscape degradation.  Vegetation can hold soil in place and reduce erosion rates.  The removal of trees and plants to build roads or villages can accelerate soil loss.  The higher the altitude, the less important this factor becomes.  At high altitude, vegetation cover is much lower, but the climate is also much more arid.  At lower elevations, vegetative cover is essential to holding saturated soil in place and maintaining the slope surface.  When it is removed, the increased surface area is vulnerable to the effects of rain.  It is clear that vegetation cover and precipitation are closely linked, and affect the landscape degradation of an area.

Precipitation and Erosion

Climate is a major factor in erosion in the Himalaya.  The range has a distinctive precipitation pattern called the monsoon.  In the summer, the foothills are subjected to extremely heavy rainfall.  This causes intense flooding and landslides.  In the spring, glacial meltwater causes further erosion on the unstable slope, and avalanches are also common (Owen 1996). 

Fluvial erosion plays a large role in the degradation of the Himalaya Mountains.  From the Indo-Gangetic Plain (a large floodplain made up of sediments 10 kilometers deep) to the high, crystalline Himalaya, water erosion is a major factor.  As these mountains uplift (at incredible rates, about 6 millimeters per year!) they are cut down almost as quickly, creating immense, beautiful valleys and river terraces. The rivers carry this sediment out of the range.  You can see the different sediment loads that the Zanskar and Indus Rivers (pictured above) are carrying.   The Zanskar River is a muddy brown color, indicating that its sediment load is high, and particles are suspended in the water and being transported out of the range.  The Indus River is a bluer color, because the sediment did not remain suspended but was deposited along the river bed.  Thus, I would say that the Zanskar River has more erosive power than the Indus, because of its more effective transport. 

It is hard to measure overall rates of erosion, because a mountain range has so many variable parts.  However, using biotite dating in gneisses, UC students and faculty dated strath terraces and concluded that on range margins, erosion could be up to 15-20 mm/year.  That is incredibly powerful erosion.

At higher altitudes, glacial incision is the major erosive control.  As the glacier moves slowly with gravity, it scrapes along the bedrock surfaces, and the glacial melt incises the substratum.  Glacial erosion creates characteristic U-shaped valleys, like the one pictured to the right.  Glacial lakes that were dammed by ice can catastrophically fail in an outburst flood. Snow and rock avalanches also often coincide with glacial landscapes.

Mass Movement

It is hard to tease out one causal force for a landslide event.  Movement along a fault can set into action large scale landslides.  A landslide is affected by tectonics, but also by precipitation.  This may have been climate driven; the precipitation gradient was high around the time these landslides occurred.  A study from 2004 used sediment influx measurements to make inferences about the role of precipitation in large mass movement events.  They found that rainfall increases pore water pressure, “priming the
landscape for larger and more frequent slope failures,” and that these failures often occur late in the monsoon season (Gabet et al., 2004a).

Even though the slope failures pictured seem so big, they are actually not as erosionally “effective” as fluvial sediment transport.  This is because, although a huge mass of material was moved, it was not moved far.  After the fall (both were dated to about 8,000 years ago), this mass has remained in the exact same place. 

Slope instability is the result of many factors, including bedrock and soil type, angle of slope, vegetation cover, and influence of climate.  In the talus cones pictured above, all of these variables come into play.  Above the cones one sees a pro-talus rock glacier.  During the winter this entire slope is covered in snow, and interstitial ice forms.  The melt from this ice leads to even more slope instability. 

           

Future Research

There is so much more to be explored in understanding the relationship between precipitation, tectonics, climate, anthropogenic influence and erosion in the Himalaya.  This research is becoming especially important because of global climate change and all of its implications.  Understanding the present is the key to the past:  If scientists can piece together an erosional history of the mountain range, then we can better recognize erosional patterns of the present, and better predict erosional patterns of the future.  What is the effect of climate change on Himalayan landscape degradation, and what does that mean for the people of this range?  What predictions can we make about future regional geomorphology?  It is an immense mountain range, with immense possible resources and research opportunities.