Rivers change immensely on their journey from Source areas (where they start) to their finishing point at their mouths. There are 4 major concepts here;

The drainage basin
The water cycle
The changing characteristics of rivers from the mountains  - LONG PROFILE
The changing valley shape -CROSS PROFILES

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, as well as the water table. Drainage basins are separated by watersheds. A drainage basin is an example of an open system because it is open to inputs from outside, such as precipitation, and is responsible for outputs out of the system, such as output of water into the sea and evaporation of water into the atmosphere. You can see a diagram of the drainage basin below.

The water cycle
This is a series of processes by which water is evaporated from the sea and eventually condenses and precipitates over the land.  It is the main input to the drainage basin and you can read detailed notes on this process at the Met Office's website.

Changes in river variables/characteristics - LONG PROFILES

The long profile of a river is a way of displaying the channel slope of a river along its entire course. Therefore, it shows how a river loses height with increasing distance towards the sea.

The diagram above shows how the long profile changes downstream. In the upper reaches (also known as the source) the gradient is at its steepest because of vertical erosion. This changes further downstream as lateral erosion (side to side) from the river and deposition start to replace vertical erosion as the dominant process. Finally, the river really flattens out as it approaches the mouth as deposition become dominant. In addition, along the upper part of the long profile or Thalweg there is more turbulence, lots of bed load in comparison to discharge and lots of roughness and friction. As more streams and tributaries join the river, roughness decreases, discharge and velocity increases and the erosive power of bed load will decrease. As a result the gradient of the river will generally decrease creating a concave long profile with distance down stream, and deposition serves to enhance this phenomenon further. Some of the key features of river channels are shown below;

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 below summarises the changes that occur;

Velocity increases as more water is added to rivers via tributary rivers.  This means that less of the water is in contact with the bed of the river and the banks so there is less energy used to overcome friction.  Hence rivers flow progressively faster on their journey downstream.

Width and depth increases as more water is added from tributaries.

Gradient (the slope of the land) decreases as rivers flow because the river meanders across the land rather than erode into it and follow a straight path as it does in the source.  This means it covers a decrease in height over a longer distance the further downstream you get.

Stone size decreases downstream and the stones get rounder and smoother as rivers erode the rocks progressively as the stones are transported downstream. The processes of river erosion operate here.

Changes in valley shape - CROSS PROFILES

The diagram on below shows the major changes downstream.

In the source area the drainage basin (an area of land drained by a river and its tributaries) contains V shaped valleys and waterfalls, and the dominant processes are erosion.  Erosion tends to be vertical (straight down into the land).

In the middle section of the drainage basin the river starts to erode laterally.  This section contains meanders and Ox bow lakes, and the river creates a flood plain often with Levees.  Here, Material is deposited and erosion can also occur.

In the lower drainage basin deposition dominates as a river enters a sea or lake, the valley is at its widest and deltas and estuaries are major landforms and habitats