Changes in a river from source to mouth Learning Objectives: Describe the changes in river features from source to mouth Explain the reasons for these changes using the Bradshaw model

The drainage basin This is the area of land drained by a river system (a river and its tributaries). It includes the surface run-off in the water cycle. Drainage basins are separated by watersheds. You can see a diagram of the drainage basin below.

https://www.youtube.com/watch?v=nNAYe8vp3aQ

River Feature Upper – near the source Middle Lower – near the mouth

The diagram on the next slide shows how the long profile changes downstream. At the source the gradient is at its steepest because of vertical erosion. There is more turbulence due to lots of bed load in compared to discharge (the amount of water flowing in the channel per second), there is lots of roughness and friction. This changes further downstream as lateral erosion (side to side) from the river and deposition start to replace vertical erosion as the dominant process. As more streams and tributaries join the river, roughness decreases, discharge and velocity increases and the erosive power of bed load will decrease. Finally, the river really flattens out as it approaches the mouth as deposition become dominant.

Feature – Slope Angle In your table describe slope angle at each part of the rivers long profile.

Feature – River Valley Cross Section In your table describe the shape of the river valley cross section at each part of the rivers long profile. Now draw the shapes across your table under your writing.

Feature – River Shape (depth and width of channel) In your table describe the depth and width of the river channel at each part of the rivers long profile. Now draw the channel shape in your table under your writing. Finally draw a line across your table to start a new section.

Upper Course A narrow steep – sided valley where the river is very thin and very fast moving

Middle Course The valley is much wider here having a flat floodplain and meanders

Lower Course A very wide and flat floodplain the river is also very wide here and it moves slowly

The Long profile of a river

The Bradshaw Model This model shows what should happen to various river characteristics as a river journey's from source to mouth along its long profile.  I have modified the model a little to add some extra characteristics. There are many changes that occur in rivers as a result of changes within the water cycle and drainage basin. The diagram on the next page summarises the changes that occur;

Feature - River bedload (material found on the bed of the river channel) In your table describe the size and shape of the bedload at each part of the rivers long profile. Now draw the bedload across your table under your writing. Finally draw a line across your table to start a new section. Upper Middle Lower

Feature – Water Velocity (How quickly the water is moving ) Where do you think the water will be moving at its fastest e.g. near the source or mouth? What is your reasons for this decision? The actual answer is slowest and the source and fastest at the mouth, but why/how? Reasons : Friction between the water and bedload is high at the source and ,low at the mouth. The bed load at the source often blocks the water flow causing turbulence. Add this information to your table

Erosion – the removal of material from a place River processes The reasons for the changes in Bradshaw's models are down to 3 processes that shape our landscapes: erosion, transportation and deposition. Erosion – the removal of material from a place Abrasion – the material carried by the river wears away the bed and banks. Hydraulic action – the power of the water forces air into gaps in the banks and weakens them so they eventually collapse. Attrition – the particles carried by the river (its 'load') are bashed against one another, making them smaller and rounder. Corrosion/solution – particles are dissolved.

Transportation – the movement of material in the river Traction – rolling large stones along the bed of the river. Saltation – bouncing smaller particles along the bed. Suspension – sediment that floats within the river flow. Solution – minerals dissolved in the water.

Deposition – material is dropped by the river When the velocity of a river falls, it has less ability to carry its load. The river then starts to drop the material, starting with the largest particles. The velocity can fall for many reasons, including when: the river enters the sea or a lake there is less water entering the river there is more sediment in the river Using the ideas of erosion, transportation and deposition try to explain the changes in the features of Bradshaw's model Examples: The reasons why sediment becomes smaller from source to mouth is because of..... The reasons why the river channel becomes wider and deep from source to mouth is because of ..........