Fluvial And Flooding Processes: Geomorphology In Boreal Regions
Boreal regions of the Earth are found in both the Northern and Southern hemisphere, usually below arctic or sub-arctic regions. It is these areas of the earth that are dominated by both ice and water. The temperatures there vary enough to sustain glaciers and also vast boreal forests with a distinct seasonality between winter and summer. Water, snow, and ice all play specific roles in various fluvial and flooding processes in this region, and flooding is the largest factor for transport and deposition of sediments throughout a landscape of this nature.
There are two categories of flooding to concentrate on. The first is seasonal flooding. This process is caused by the collection of snow and ice over winter and the melting over the summer. The second is caused by random outbursts from glaciers. Water becomes trapped in some form by an advancing or retreating glacier, builds pressure, and then erupts. In both categories, seasonal or glacial, the flooding has a large effect on the landscape around, and in some cases can be devastating.
Fluvial Processes During Seasonal Flooding
Seasonal flooding is the simpler process of the two. During winter months, water collects in the form of snow and ice across the countryside. The ground will also become saturated with water, and a frost will develop down into the soil, freezing the ground in place. Geomorphically, the region is in stasis with little change due to water processes. However, in the spring or summer when the temperatures consistently reach the melting point, this stored snow and ice begins to melt and an overflow of water occurs. Because the ground is already saturated with water from the thawing frost, the excess water from the snow on the ground flows more directly into the rivers causing flooding for small basins. At the same time, seasonal precipitation adds to the accumulating water, and rivers overflow their banks.
This flooding adds energy into the system according to certain fluvial laws, and sediment is transported downstream much more rapidly than any other time. Other effects of this seasonal flooding are increased debris flows, including land and mudslides. The continual freezing and thawing of frost expands and contracts the soil it is in, not only helping to sustain hill-slope transport but loosening the ground making it weaker. Because the ground already has water in it from thawing, any extra precipitation can cause the ground to reach a critical point, leading to mud and rock slides. There are areas that have been known to have mud-slides on slopes of only 1 degree.
Avalanches follow this seasonality also, although they are not caused in any way by flooding. In warmer months, the snow begins to melt, boring down into the basal snow weakening it. After certain criteria, depending on slope and the thickness of the overlying snow, the pressure will release sending an avalanche down the side of the mountain. This, in turn, can cause flooding by possibly blocking a river or even by adding a new source of water to the area.
The last of seasonal processes must be evaluated on a large time scale. This is the thawing of permafrost in the arctic or sub-arctic regions around the boreal zone due to global warming. It has the opposite effect that one might think. As the permafrost thaws, the ground becomes unstable, and indentations in the ground are formed. The water from the thawing ice then collects in pools and evaporates away leaving an unusually dry ground for the area. As the climate warms, the boreal zones expand into the sub-arctic zones, and new forests overtake the land. But with little water still in the ground, a drought forms, drastically increasing the risk of a forest fire. In the past decades, large forest fires have increased in Boreal regions such as northern Russia and Siberia, changing the landscape and affecting the ecology of the regions.
Fluvial Processes During Glacial Outbursts (Jökulhlaup)
The second type of flooding occurs from glaciers. Random outbursts of large amounts of water from the glacier are called jökulhlaup. This random flooding is caused by built-up pressure underneath a moving glacier and can present itself in many different forms. It is important to keep in mind that the glacier must be moving if it isn’t this build-up cannot occur because the water inflow and discharge are in a steady state. Oddly enough, the direction of the moving glacier plays a large role in what kind of outburst occurs, although there are many other factors.
There are three types of jökulhlaup. One is when a piece of the glacier restricting a glacier lake breaks apart, and the lake drains almost instantaneously down into the valley below. This type is the most well known because it is observable and slightly more common. This first form of an outburst, because it originates from the surface, only moves gravel sized rocks and small boulders already found on the valley floor ahead of the glacier.
The second type consists of a large volume overflow. They can erupt from either the glacier margin, where the ice meets land on the sides of the valley, or from the top part of the glacier, above the snout. This type of outburst occurs most often for advancing glaciers, although this process can occur in retreating ones. As the glacier moves forward, areas on the valley bed cause large amounts of friction with the ice breaking apart the bedrock and transporting large boulders down valley inside the ice mass. These sediments and boulders carried by the glacier can lodge themselves inside the channels through which glacier water travels and block the paths.
Once blocked, the water must find another way to move, or it just builds up pressure underneath the ice. When the pressure is high enough, the trapped water will force itself up and over the boulders blocking its path causing a flow up into the glaciers. As the water moves upward, the cracks and crevasse in the ice enlarge and water will pour out of the surface of the glacier, far higher up in the valley than where the glacier actually ends and carry down large amounts of water, snow, and ice. The type of bedrock it picks up is the finer gravel sized bedrock already exposed ahead of the glacier, and the flow quickly turns into a flooded debris flow. Transport of rock for this type of jökulhlaup can go on for many miles ahead of the glacier head.
The final type of outburst occurs basically the same way. The difference is that the pressure build-up occurs closer to the front of the glacier where the ice is not as thick. This is more common in a retreating glacier. In this area, instead of the water forcing itself up through the ice, a form of hydraulic jacking occurs and the whole ice sheet can be lifted up. This produces lateral movement of the water below the ice. This allows the water to bypass the blocked channels, and at a critical moment, will burst forth from the front bottom edge of the glacier in a wide shallow flow. This type of flow carries the large ground up boulders from underneath the glacier and usually does not travel very far. It moves enormous sized rocks and boulders hundreds of yards ahead of the glacier but does not pick up the smaller gravel rocks.
In fact, many observed large outbursts from glaciers produce the latter two types of jökulhlaups. Though there is not much direct evidence for the mechanisms that drive these floods, many different theories have been induced. In regions like Iceland, volcanism and heat coming through the earth are thought to drive these processes. While in New Zealand, where there is no volcanic activity near the glaciers, processes such as negative slope and friction are thought to drive the glaciers and outbursts.
In a final sense of completeness, both types of flooding, seasonal and glacial, are closely related because of large rainstorms, which occur more often in the summer, and are what drive not only the flooding in rivers but also necessary the glacial outbursts. All outbursts have a strong correlation with a large storm in the area beforehand.
The Boreal regions of the earth are really exciting with exploding glaciers, avalanches, and flooding. It is important to remember though, that in some way all these processes occur because of water in one of its forms. It is water that drives not only the evolution of the creatures, but also the evolution of the earth, and the boreal regions are where all extremes of geomorphic water processes take place. It will play an important role in the future with the changing climate and will be a place of many studies in the future.