There are 3 main features you could draw: waterfall, meander of delta. We recommend you do a waterfall for this type of question. You can find pictures of all of these in a Geography text book or good encyclopedia. Here are the main facts and labels you need to know:

Waterfall:
The rock on the top of the waterfall is a hard rock, it is good to give an example such as Limestone
The rock underneath and behind the waterfall is a soft rock which can be easily worn away. Again, a good example of this is shale

HOW THE WATERFALL FORMS:
A step is present in the soft rock which forces the water downwards
The water starts to erode as it is pushed down and creates a plunge pool
The water keeps eroding and the waterfall gets bigger
At this point the water starts undercutting the soft rock behind the waterfall
This process continues and hard rock is broken off as the waterfall retreats
Spray from the waterfall undercuts more
A good example is Niagra Falls.

Meander:
Meanders may seem very simple but it is important you can label them and get all the facts in for maximum marks
If something is in the way of a stream or river it has to go round it for example a tree or a rock
After this first bend, the river hits the outside of the bend with more and more force to erode the river bank and enlarge the meander
The eroded material is deposited on the inside of the bend >>> label this as the river beach
This process continues >> The meander gets bigger and bigger until finally the river cuts through and meets at the other side.
This is an oxbow lake.

Delta:
Deltas are formed at the end of rivers (river mouth) as it flows into the sea
Rivers carry a lot of sediment (mud)
The river flows into a calm sea
This slows the river down
Which makes the river drop its sediment
This deposited sediment builds up over years creating a delta
The river is forced to split up
The smaller rivers made by deltas are called distributaries
Examples of Deltas are Nile Delta, Ganges Delta