Question

Consider spatial variability in rates of erosion of the landscape and sediment loads in rivers, including the methods used to measure rates and loads, and the relative influence of climate vs. tectonics vs. other controls. How is the system size involved?

Answer 1

Rivers form by the confluence of several streams within a watershed. These streams in turn form by water collecting as it flows down the slope. Along these slopes, water moves fast and has a lot of energy. This allows it to carry larger sediment sizes including boulders and smaller and at a faster rate. Thus, the erosion rate among the mountains where the river forms is faster.

The river carries this sediment, and when it comes onto plains, its velocity reduces, and it no longer has the energy required to carry larger sediment sizes and it begins to deposit larger sediment sizes. This leads to the formation of alluvial fans at the base of the mountainous region.

At the plains, the river starts meandering, and mainly contains fine sand and finer sediment sizes, and it begins depositing these on the inner banks, forming point bars. Near the sea, the water moves the slowest, and if it laden with sediment, even fine silt sized sediments get deposited, and deltas form.

In high rainfall regions, the rivers have more water flowing through them, and this gives rise to more voluminous rivers, which can carry larger sized sediments, whereas in more arid regions, the discharge is much lower, and most erosion is wind dominated.

When continents rise, the base line remains constant, and the river continues to dig in deeper. This leads to the formation of features such as antecedent topography where rivers flow through mountains instead of around them, and incised meanders where meandering streams form steep valleys which they should not be able to. The most famous example for this is the Grand Canyon.

The ultimate effect of all these controls depend on both the spatial and temporal scales being considered. It takes millions of years for even a high energy stream to carry large boulders from the top of mountains to the base. This is because these boulders move only in high flow conditions that exist just after periods of heavy rainfall. Thus, if heavy rainfall in such a region is rare, or if it never occurs, the sediment load within a region will be very small. The boulder will be carried forward, as long as the stream has a high discharge. If the boulder moves into a region of lower precipitation, this is where it will be deposited. Thus, the distance the boulder is carried depends upon the scale of high precipitation. Similarly, local faulting will cause offset streams, whereas large scale plate rising will cause antecedent topography as mentioned earlier.