A Level Geography- River Exe

Where is the River Exe located?

The River Exe flows 82.7km from its source in the hills of Exmoor, through Tiverton and Exeter to the sea at Exmouth on the south coats of Devon.

How many tributaries does the river have?

The River Exe has 5 tributaries: Haddeo, Culm, Clyst, Barle and Creedy.

What is the impact of the tributaries?

It has an extensive network of tributaries and a high drainage density; this is because of the lots of tributaries in a small area which will lead to rapid surface runoff.

What are the physical features of the drainage basin?

The upper catchment covers 601km² in area.

Its maximum elevation is 514m in the north.

The land is much flatter in the south (lowest elevation is 26m).

Therefore, the drainage basin will have a low infiltration rate and increased surface run-off. When there's lots of precipitation there will be a high peak discharge.

What are the geological characteristics of the drainage basin?

84.4% of the basin is underlain by impermeable rock (Devonian sandstone). This encourages the extensive drainage network.

Impermeable rock means very little infiltration and lots of surface run off. Water will be stored in the soil and plants before quickly flowing into the river and tributaries.

What is the land use of the drainage basin?

Most of the land is agricultural grassland (67%) with some woodland (15%) and arable farmland. On the high ground of Exmoor there are moor and peat bogs (3%).

The grassland and woodland will store water and reduce the surface run off due to interception and stemflow. The peat stores the water so it's transferred very slowly. This will decrease surface run off and increase infiltration.

What is the water balance for the Exe catchment?

precipitation (1295mm) - evaporation +/- water storage (451mm) + runoff (844mm)

How does precipitation impact the drainage basin?

Rainfall is high in the study area, particularly over Exmoor. Much is absorbed by the peaty moorland soils. However, if saturated or where drainage ditches have been dug, water can flow off the hills rapidly.

How much run off is part of the water balance?

65%

Why is run off relatively high compared to the rest of the UK?

The impermeable bedrock reduces percolation & baseflow, causing the soils to become quickly saturated and limit infiltration rates.

Drainage ditches on Exmoor further reduce the amount of soil moisture storage available. A drainage ditch is a man-made feature which has been created in order to drain an area of land.

The Exe responds relatively slowly to rainfall events.

After flooding, discharge recedes slowly (reflecting the rural nature of the catchment - 82% grassland or woodland).

What is a human activity on the drainage basin?

The Wimbleball Reservoir

In 1979, the River Haddeo, an upland tributary of the Exe, was dammed to create a reservoir covering 150 hectares. The reservoir supplies water to Exeter and East Devon. The dam regulates water flow to ensure a steady flow regime throughout the year.

What is a positive impact of the reservoir and dam?

It prevents large peaks & troughs of water discharge (making flooding & drought less likely).

How has peat restoration impacted the drainage basin?

The Exmoor Mires Project works to restore 2000ha of peat bogs (mires) by blocking drainage ditches with peat blocks or moorland bales. This increases water content and returns the ground to the saturated & boggy conditions that would normally occur.

This helps to retain carbon stored in the peat.

What are the negative impacts of human activity?

For decades, drainage ditches have been dug in the peat boys of Exmoor to make it suitable for farming. This has increased the speed of water flow to the Exe (reducing water quality due to increased silt transportation).

Peat has also been dug as a fuel, leaving behind ugly scars on the landscape. As the peat dries out, decomposition occurs- releasing carbon in the form of CO2 and methane.

What are the advantages of peat restoration?

-Increased water storage. Water transfer is slowed, increasing storage capacity and ensuring a steady supply of water throughout the year.

-Improved water quality. Slower throughflow means that less sediment is carried into rivers. This makes the water cheaper to treat and is good for salmon.

-More carbon storage. Peat is basically carbon & water (a major carbon store). It naturally absorbs carbon from the atmosphere & stores it. Dry peat releases CO2 through oxidation.

-Improved education, leisure and recreation opportunities. Peatland habitats are very biodiverse, with many species of plants, birds, butterflies and insects.

-Improved grazing and water supply for animals. Animals benefit from having year round drinking water, as well as improved grazing during the drier parts of the year.