The upper course of the river starts at the source of the rivers channel. The upper course of the river is shallow and has a narrow river channel due to vertical erosion caused by hydraulic action and corrosion. It also has an uneven steep river bed due its bedload as it is large boulders as they have rough edges. This then gets transported through the upper part of the river by traction. In addition some of the bedload also gets transported by suspension and little in siltation. As the river flows through the upper course of its channel the shape and size of the channel changes which result in distinctive landforms; these help to develop the landscape of the river during its course. These landforms are frequently caused by erosion and deposition. Vertical erosion in this highland part of the river helps to create steep sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges. Waterfalls and rapids occur when there is a sudden change in the gradient of the river as it flows downstream. They are most commonly found in the upper part of the river where there are marked changes of geology in the rivers valley. In the upper Teesdale an outcrop of igneous rock called the whin sill caused the formation of the High Force Waterfall which is 22cm high and is the tallest waterfall in England. The waterfall is formed by The bed of the river below a waterfall contains boulders eroded by splash back from behind the waterfall, and some blocks of rock from the collapse of the hard cap rock. Over a very long time the process of undercutting and collapse, causing the waterfall to retreat upstream. This creates a gorge within the rivers landscape.