How Rivers Change Landscapes

Extensive Flooding In Co.Clare

This scene from County Clare will not be an uncommon sight anymore due to the rise in sea levels, extreme weather conditions and increased winter rainfall.

Courtesy of the Department of the Environment, Heritage and Local Government

Extensive Flooding In Co.Clare
Courtesy of the Department of the Environment, Heritage and Local Government

Extensive Flooding In Co.Clare

This scene from County Clare will not be an uncommon sight anymore due to the rise in sea levels, extreme weather conditions and increased winter rainfall.

Courtesy of the Department of the Environment, Heritage and Local Government

Enlarge image

Powerscourt Waterfall

To preserve the natural beauty of our island it is essential they we all do our part to contribute to the environment and its conservation

Wikipedia

Powerscourt Waterfall
Wikipedia

Powerscourt Waterfall

To preserve the natural beauty of our island it is essential they we all do our part to contribute to the environment and its conservation

Wikipedia

Enlarge image

Rivers are one of the most important agents in shaping landscapes because their widespread presence ensures that no region on Earth is totally excluded of landforms developed by fluvial processes.Rivers do much more than transport water and sediment. They also alter a non-descript geologic setting into distinct topographic features. This occurs primarily because the transportation of sediment-laden water is capable of pronounced erosion. Conversely, some fluvial landforms are created by sediment deposition. Some fluvial features are entirely erosional, whilst others may be entirely depositional. In depositional cases, the topography is comprised of sediment that buries some underlying surface that existed prior to the introduction of the overlaid sediment. However, many fluvial features are a combination of both erosion and deposition.

Two of the most visible ways in which rivers shape the physical landscape are in the formation of river valleys and floodplains.

River Valleys

The majority of river valleys share a common feature in that their characteristics are the result of river erosion; i.e., rivers produce the valleys through which they move. The primary shape of a river valley is determined by events that occurred during its developmental history and the characteristics of the underlying geology. Whilst not always visible due to the slow nature of change, river valleys are constantly being re-shaped, primarily due to processes of erosion.

During initial valley development, a valley's topographic relief tends to increase as rivers use most of their energy in vertical entrenchment/downcutting. River valleys tend to be narrow and deep, particularly in areas where they are cut into unfractured rocks with properties that resist erosion (most igneous rocks, some sedimentary rocks such as quartzites, and silica-rich metamorphic rocks). Abrupt changes in river and valley bottom gradients, including waterfalls (e.g. Powerscourt Waterfall, Co. Wicklow), are common in the initial developmental phase. As vertical downcutting continues, however, rivers gradually smooth out the gradient of the valley floor. Under such circumstances, more energy is expended laterally than vertically, and the valley floor of the river progressively widens. Consequently, most river valleys change over time from narrow forms to broader ones, the shape being ultimately dependent on the erosional rate, which itself is a feature of rock type and rock structures.

One of the most well known river valley forms are gorges or canyons that develop from enhanced river entrenchment caused by recent tectonic activity, especially vertical uplift. Canyons and gorges are still in the early stages of valley development. They range in size from narrow slits in resistant bedrock to large channels. Where underlying bedrock is comprised of flat-lying sedimentary rocks, regional tectonic uplift creates elevated plateaus and simultaneously enhances the erosive power of existing rivers, a characteristic known as rejuvenation. Vertical entrenchment creates different valley styles depending on the river's size and the extent and rate of uplift. The Grand Canyon of the Colorado River, in the U.S. formed as a result of uplift of the Colorado Plateau (Britannica, 2018).

Flood Plains

Floodplains are one of the most common fluvial features and are typically located along every major river and in most large tributary valleys. Floodplains can be characterised topographically as relatively flat surfaces located adjacent to river channels, which occupy much of the area constituting valley bottoms. A floodplain's surface is underlain by alluvium deposited by the adjacent river and is partially or totally inundated during flood events. Consequently, a floodplain is not only created by but also serves as a core part of the modern fluvial system.

When rivers flood, vertical accretion occurs incrementally along the adjacent land. However, the extent of a floodplain constructed entirely by deposition during a flood event normally increases at a progressively decreasing rate. This arises because as the height of the floodplain surface increases relative to the river channel floor, the river height required to overtop the banks also increases. Consequently, the floodplain surface is inundated less often, and the growth rate of flood plains decreases. The initial phase of floodplain elevation by vertical accretion is therefore quite rapid because flooding occurs frequently. It is generally accepted that 80-90% of floodplain construction by vertical accretion occurs in the first 50 years of the process. A 3m thick overbank deposit would therefore probably take several thousand years to accumulate. In this way, floodplains are constantly being reshaped as the rate of accretion changes. Over time, levees (i.e. ridges of deposition) may develop adjacent to the river banks, and may also act as a natural flood defence along the floodplain in periods of high river flow (Britannica, 2018). Moreover, as floodplains are extremely fertile and nutrient rich they play a vital role in agricultural production. This indirectly alters the physical landscape further as humans modify the surrounding landscape to capture a floodplain's benefits.

In Ireland, the River Shannon catchment does not fall significantly along much of its course (from Lough Allen in Leitrim to Killaloe in Clare it falls by only 17m). This results in an extensive floodplain and a slow, meandering course for most of the river's length. One-fifth of Ireland's land mass drains inwards through various lakes, rivers and streams in to the River Shannon system, including through a series of small rivers and lakes. Due to its relatively flat course, the river produces valley topography in only a few places. Summer water levels over most of the Shannon's drainage area are only 1m or less below the level of the surrounding landscape so any increase in water levels causes extensive flooding along its floodplain (NPWS, 2002).