The changing landforms down a river's long profile

Trends down the long profile: As you move down a rivers course all the processes above will change in some way.

• Discharge - this will increase as you go downstream as will the velocity because more water is being added to the channel.
• Erosion - will increase with the discharge. Is more vertical upstream and lateral downstream.
• Transportation - the rivers capacity goes up downstream.
• Deposition - there is a lot of deposition upstream where the discharge is low and at the mouth where the sea or a lake the river flows into reduces the velocity.
• Bedload size - becomes smaller downstream

Erosion, transportation and deposition all vary with velocity, discharge and hydraulic radius (fig.1). These in turn are affected by rock type, long term climate change, isostatic change and human intervention.

An example of a long profile: The River Derwent in the lake district, England. This river Is a good example of erosion, transportation and deposition at work in a river.

The source:

Its source is in an area of high relief with steep V shaped valleys. The river here is narrow with lots of resistance and the erosion is mainly vertical. The bedload size is large and there is also a waterfall and some rapids at Taylorgill Force.

The mid section:

At this point the river widens out. There is more lateral erosion, the river meanders in places and it has a flood plain with a clearly visible bluff line.

The gorge:

At one point the River Derwent encounters harder rock and higher relief once again. This causes it to revert to vertical erosion again making it loose the floodplain and meanders to follow a narrow gorge through the rock.

The mouth:

Once past the gorge the river widens out again with a large floodplain and meanders in a broad valley. The lake it flows into starts to slow the velocity of the river causing deposition and so it has a delta at its mouth.

The different landforms:

• Interlocking spurs (fig.2) - near the source of the river where there is high relief a steep valley is formed which has to take the easies route between hills that juts out. This forms interlocking spurs.
• Waterfalls (fig.3) - are caused by changes in the rock that a river flows over. If a river flows over a hard rock followed by a soft one the soft rock will wear away faster leaving the hard rock sticking out. Over time the water falling from the band of hard rock cuts a plunge pool in the soft rock. This undercuts the hard rock which eventually collapses back and this process can happen over and over again causing the water fall to move back over time.
• Rapids (fig.4) - Upstream where rivers discharge is low the water is forced to flow over shallow rock and bedload. This creates lots of white water turbulence such as eddies and stoppers as the obstacles slow the water.
• Gorges (fig.5) - These are steep sided valleys where the vertical erosion of the river has cut straight down into the earth. They can be caused by waterfalls retreating backwards and leaving the lowered river path or by the roofs of caves collapsing in to leave the river running at the bottom of a newly made cutting. Gorges can also be formed by isostatic change. When the land a river flows over rises the river will erode vertically down to try and stay in the same position. This will leave a gorge where the land has risen up around the river.
• Meanders (fig.6) - the main feature of the lower end of a long profile. They are formed because of the fluvial tendency to not move in a perfectly straight line. Once the river has the slightest bend in it erosion transportation and deposition will work to give the river meanders of greater and greater sinuosity. Because of centrifugal forces water flows fastest round a bend on the outside. A higher velocity means more erosion and deposition which makes the outsides o the bend deeper. The lateral erosion eats away at the bank, continually extending the bend in the river. On the inside the water flows slower, this causes deposition and the rivers material is dumped there. This extends the inside of the bend in the direction of the outside so that in time the whole meander has a greater amplitude. In between the bends the river bed is shallower causing turbulence in the water called riffles.
• Point bars (fig.6) - form on the inside of the river bends when the load is deposited and stick out, growing longer as more material is added.
• Ox-bow lakes (Fig.7) - these as bends in a river that are cut off from the main channel. They are formed when a river meanders with such sinuosity that in a time of flood the water can break through forming a new channel that isolates the old meander. Over time it will fill with silt and disappear.
• Flood plain (fig.8) - When a river passes bankfall it will spill over onto its flood plain, an area of flat land surrounding the channel which is covered when there is a flood. A flood plain can be made wider by the lateral erosion of meanders and they are often built up by rich sediment that is deposited every time the river floods. The clearly definable edge of a flood plain is called the bluff line.
• Levees (fig.8) - The rivers load in a flood is usually deposited near to the river bank. This makes the sides build up higher with every flood until the rivers water level can actually be higher than the rest of the floodplain while the levees at the sides of the river hold all the water in. Artificial levees are built by man to help prevent flooding.
• Delta (fig.9) - Formed at the mouth of the river when it reaches a lake or a sea that doesn’t have very strong currants to take the sediment away. As the river is slowed by its impact with the standing water it drops almost its entire load. This over time will build up and extend the river out from the land on its own delta.