I expect the size of the river channel to increase as we go downstream. Near the source of a river, you will expect it to be in an upland area with a narrow channel. It would have large boulders and large angular bedload which will, therefore, result in it being a rough channel.
Factors that I expect to help the stream increase are tributaries that will bring more water and discharge it into the river, increasing not only its channel but velocity of the river as well. Tributaries also bring more bedload into the main river and so increasing the amount of bedload. Tributaries are one of the main sources that discharge water into a main river. Surface run-off will also bring in more water from the moving water on the surface as the ground is saturated and the water cannot sink into the ground so, therefore, it will flow into the river. Ground water collected from permeable rocks will also add more water increasing the river channel, velocity and bedload as well. Throughflow will discharge more water into the river. This downhill movement of water through the airspaces in the soil is also another way in which the size of the channel will be increased. Infiltration will also help increase size of the channel as the water soaking into the air spaces in the soil will be released into the river. The thicker the soil the more the water added into the river. Human discharge is another factor that adds water to the river channel. Human waste will usually be disposed through pipes and though it adds more to the channel it also pollutes the river with the human waste it contains.
As the river flows downstream they are a lot of processes and types of erosion that occur and help the size of the channel increase. River erosion will cause the river to erode material from the landscape. This happens in different ways. Hydraulic action will dislodge particles in the river and banks. During a flood disaster the hydraulic action will so powerful that it will dislodge a large amount of particles that will form a river cliff. Corrasion will also increase the area of the channel. This will be caused by the bedload being transported by the river which will rub against the river bed and banks causing other particles to become dislodged and increasing the area of the river channel. Corrasion also referred to as Abrasion also contributes to the formation of a river cliff. As we go downstream the river continues to erode material from its channel, through vertical erosion, but it also starts to erode sideways- lateral erosion. The amount of water in the river has increased, mainly from smaller streams or tributaries, so the river has become more efficient in eroding and transporting material.
I expect the velocity of the river channel to increase with distance downstream. One of the factors that will influence this is gravity that will put force on the river bed and aimed down the slope causes the water to flow. On the other hand friction is the force that opposes the downstream movement of water and acts as a resistance between the water molecules and the river bed and banks. So friction will be slowing down the velocity of the river. The zone of friction will have a greater force upstream the source. This will be because of the large boulders, large angular bedload, narrow channel and the river has less water which allows a greater force of friction reducing the velocity of the river and making it inefficient. Rivers near the source are often in mountain areas and usually confined to the bottom of a narrow, steep sided v-shaped valley. The shape is as a result of the vertical erosion by the river and downward movement of the material from the valley sides due to weathering and mass movement. This material is then washed away by the river which is able to transport large amounts of rock waste especially in flood conditions. Besides being confined to a v-shaped valley a mountain stream is often seen meandering around interlocking spurs of hard rock which the river does not have the energy to erode quickly, this is because about 95% of the river energy is used to overcome friction. Lots of turbulence will be evident and the river will be described as inefficient.
As I go downstream the channel gets bigger and deeper due to the large amounts of water brought in from other sources like tributaries, throughflow, infiltration, groundwater and human discharge. Hydraulic action, corrasion and vertical erosion will increase the size of the channel making it bigger and wider. Lateral erosion will erodes the river bed making it deeper and all this increases as we go downstream. This means an increase in the velocity of the river as the water will now be flowing at a greater force due the large amounts of water and the bigger, deeper and wider channel and friction will have a less effect on the velocity of the river as the river has a more efficient channel. The water is free and can now move faster. The increase in velocity will bring about an increase in erosion and transportation of material. This can be seen when the maximum velocity is directed towards the outside of the bend as the river flows around a meander causing erosion to take place. This will undercut the river bank and as it collapses a river cliff is formed. As the water then swirls beneath the surface and as friction reduces its capability to carry material, the eroded stones are deposited on the river ‘beach’ often exposed at low water. Continued increase in velocity will cause continued erosion of the outside of the meander which will in due course wear away the valley sides so altering the shape of the valley from meandering to a straighter course.
As I go further downstream, the river start to flow over fairly flat lowland areas and continue to erode material from the valley sides and from the channel itself. The river is now transporting large amounts of water and being quite deep is much more efficient in moving material, this load may often give it a brown appearance.
I expect discharge to increase with distance downstream. Discharge is the cubic amount of water moving past one point per second. It is calculated based on two variables, area and velocity. Discharge= Area (m ) Velocity(m /sec)= m /sec. When both the area and velocity increase discharge is expected to increase as well. Area of the channel is expected to increase as it gets wider and deeper. Large amounts of water together with hydraulic action and abrasion will dislodge particles increasing the area of the channel. Vertical and lateral erosion will also increase the area of the channel as they are the main types of erosion that make the channel wider and deeper. Vertical erosion makes the channel deeper as the river erodes material from its channel and lateral erosion makes the channel wider as the river starts to erode sideways. In effect the area of the channel will increase on area. Velocity of the river is also expected to increase with distance downstream and this will be caused large amounts of water brought in from other sources into the river. The main source of water being added to river is from tributaries and others like groundwater, throughflow, infiltration and human discharge etc. now that the river is wider and deeper there is a reduced force of friction that allows water to flow faster and more freely. Increase in variables, area and velocity, will result in an increase in the discharge as well. On the other hand, if the area and velocity stay the same then the discharge will not increase or alter but stay the same as well.
I expect the bedload to become smaller and rounder with distance downstream. You will find large angular bedload and large boulders upstream. As I go downstream the bedload gets smaller and more rounded as it moves in high energy conditions. Traction rolls the large angular bedload and large boulders along the river in high energy conditions. Attrition will make large angular bedload get smaller and rounder as the stones will hit other stones causing bits to come off. The second stage is Saltation which bounces these small stones (smaller bedload) along the river bed (leap-frogging motion), causing the bedload to become even smaller. Corrosion will cause stones to hit the river banks and this will cause particles to be dislodged off the river banks and a river mat be formed. As the bedload moves downstream in high energy conditions, it erodes both the river bed (vertical erosion) and banks (lateral erosion). By the time the bedload gets to the final process of the river transport, solution, it is smaller and more rounded. The load has dissolved and is now invisible load. Corrosion also causes some minerals (although tiny) to dissolve some rocks that form the river bed and banks. As a result stones will get smaller and more rounded with distance downstream. Below are diagrams showing the four different types of river transport.
River Transport Processes