[MUSIC] You should have a pretty good idea about the ways that three sources of water in mountains, rain, snow, and glaciers can make their way down slope. Now we will look at this in more detail and, specifically, processes that control how runoff moves and some of the effects of runoff on the landscape. Runoff can move in different ways through the mountains with the common theme being that it's always trying to find its way down slope. Water may move as overland flow where the water just flows over the ground surface. Eventually forming streams, or finding it's way into one. Alternatively, it may percolate into the ground, and then eventually make its way into local streams. Or if it goes deeper, the water will become part of the groundwater system. Another component of the hydrologic cycle. Groundwater is water that fills the pore spaces and fractures in rocks and sediment. The saturated zone is where all available fractures and pore space is filled in the sediment and the rock. On top of the saturated zone is called the water table. Ground water is essentially maintaining streams and rivers in high elevation areas in winter. During the winter, when there's little rainfall, the snow isn't melting, and neither are the glaciers. So ground water is essential for maintaining flow into rivers and into streams. What controls how runoff moves across mountain landscapes? [SOUND] The steepness of the slope and the material it's comprised of is critical, of course. Steep slopes made up of bare rock, or bare rock covered in sparse soil with patchy or little vegetation, will favor rapid and intense runoff. If the precipitation event is particularly intense, or long-lasting, this type of surface flow, can easily lead to flooding. This is because there's very little time lag between the rain falling and finding its way into a stream. Vegetation plays several roles in determining how runoff moves through the mountain landscape. Leaves and branches filter the water coming from the sky by capturing it directly. By reducing the amount that hits the ground, and also by slowing the rate at which it falls. The vegetation also directly removes water from the ground through their roots. This is eventually released back into the atmosphere through transpiration. The process by which water is taken in by the plants and evaporated out of their leaves and stems. When combined with normal evaporation from the soil, this becomes part of the water cycle known as evapotranspiration. The key point to remember here is that alpine plants and forests help with water storage, preventing over saturation, and can delay runoff. If the slopes aren't as steep, and are covered in loose sediment and deeper soils, or the bedrock is comprised of certain types of porous rock, like limestone or sandstone, there's a better likelihood of infiltration. The infiltration may be relatively shallow, with the soils absorbing the water. Alternatively, it may penetrate more deeply and become part of the groundwater system. This process depends on the intensity and length of the rainfall. But if deeper soils and sediment are sitting on the surface of the rock, and there is good vegetation cover then runoff will be delayed. There is also a smaller chance of flooding and other hazards. However, if there's a lot of loose sediment with not too much vegetation on steep slopes, water can easily destabilize this surface. One of the major roles of runoff in the mountains is erosion. Shallow soils and loose sediment can be saturated, dislodged, and moved down slope to streams and rivers. If you look at a stream or riverbed in the mountains, they're often full of boulders, cobble, and gravel, not to mention a fair amount of vegetation that was destabilized upstream. This is because the slopes are steep, causing water to move faster and with higher energy, making it possible for the water to push large sized sediment down slope. Mountains with their high energy environment, exposed bedrock, loose sediment, and occasionally patchy vegetation will have high rates of erosion. So, as mountains are born and grow, the processes of erosion, led front and center by water, will work furiously to bring them back down. One day, that process will take over completely, and turn mighty mountains like the Himalayas into gently rolling hills like the ancient Appalachians. Let's apply the knowledge we've gained so far and consider more closely the runoff regime of the Bow River in Southern Alberta. The Bow River flows out of the Southern Rocky Mountains and through Calgary over the course of a year. The first graph we're going to look at describes how the stream flow varies between May and October. First, notice how seasonal it is, with very little flow in early May where the groundwater is almost the sole source of water. At this time, the river level is essentially leveled with the water table. However, flow rates increase dramatically from late July through September. You can see that there's a quick drop off in late September going into the fall. Also, notice how jagged the signal is. This is the diurnal, or daily signal, and shows how the amount of flow changes over the course of 24 hours. Why do you think the diurnal signal in this graph becomes more prominent later in the season? [SOUND] Let's take a look at another graph to help us see why this is the case. This second graph shows the contributions of snow, ice and rain to the flow of the river over the same time period. You can see that as the flow of the river starts to increase in the spring, all of the run-off that's causing the increases from melting snow. As the weather continues to warm, even though snow melt is still dominating, rain becomes more frequent. The diurnal signal that exists here is related to the snow melting strongly during the day, and strengthens as time goes on, but doesn't reach its maximum until the third contributor to the run-off becomes prominent, glacier ice melt. By mid July and early August, the contribution of rain has increased as we see an increase in convection storms caused by hot summer days. We also see a rapid increase after this time in the contribution of glacier ice. By mid August, most of the runoff that's contributing to the flow of the Bow river is coming from the glaciers at it's head waters. And the diurnal signal is pronounced. Since glaciers are such high elevations, their flow will almost come to a complete stop during the night, as it cools dramatically from daytime highs. In late summer, glaciers continue to dominate river flow, but the diurnal signal starts to fall off in September, as the days begin to cool, as well. So it should be clear now that glaciers are incredibly important for maintaining good flow in rivers and streams in mid to late summer and fall when the previous season's snow has melted and rainfall is less. Where I live in Alberta, major rivers like the Bow, the North Saskatchewan and the Athabasca are fed by glaciers in the Rocky Mountains. Prairie farmers, communities, cities, and industrial activities rely on these rivers. As warming continues and the retreat of the glaciers that feed the world's major rivers accelerates, there's a real potential for a water crisis on the horizon. We'll return to this aspect of mountain hydrology in future lessons.
