Rivers Modified by Man

Wild Trout Trust Conservation Officers spend around 1000 days a year walking along river banks and working in rivers. They are specialists in how rivers ​‘work’ in practical terms, and in assessing, improving and creating habitat in rivers for trout and all wildlife.

Check out any one of their Advisory Visit reports or this example to see the kind of things we notice and the pragmatic advice we give.

Almost all our advice and project work involves recognising when a river has been modified by man, and working out how to return natural processes to the river as far as is possible and practical. Rivers that function naturally behave more predictably in times of flood and drought, have no or very low maintenance requirements and most importantly for us, they will be wildlife corridors with good, natural habitat.

In England, ​‘physical modification’ is the most common reason for rivers to fail to meet ​‘good ecological status’ — more rivers fail for this reason than for point source or diffuse pollution.

These are some of things that we look for:

To state the obvious, water always finds the lowest point and rivers naturally find the lowest point in a valley. However, throughout history, people have moved rivers away from their natural course for reasons that include creating larger, better drained fields for agriculture, making space to build roads or railways, creating water meadows with a network of ​‘carriers’ or as part of an engineered flood management plan. 

Rivers that are ​‘perched’ above the lowest point in the valley are likely to break out their banks and try to get to the lowest point either by overtopping the artificial banks in a flood or by eroding the banks until they ​‘breach’.

A perched channel is like a caged tiger that will take any opportunity to escape.

In giving advice on perched channels, the first and best option is to allow the river to go back to the bottom of the valley, which would very likely have been its original channel. Often you can see these relict channels using google earth or the more specialised LIDAR images. Sometimes these images reveal the fascinating story of a river.

Wiggling is not exactly a technical term but it is a good description of what rivers should do. Nature very rarely does straight lines, so when you see a river that is straight and is of uniform depth over a reasonable distance (sometimes called a trapezoidal channel) you can be fairly certain the river has been modified by man.

Below left: The River Avill in Somerset. It looks like a ditch, but it is a river that has been straightened and dredged so all natural ​‘wiggles’ have been removed. .

Below right: The River Yeo has been encased in a straight concrete channel.

Many, if not all, lowland rivers in England will have been straightened and dredged at some point. Often perched channels (see above) are also dredged and straightened and for similar reasons. Gravel dredged from the river channel is useful for building roads and tracks and for building up the river banks in order to stop flood water coming out of the channel and onto the floodplain.

Rivers that wiggle, or meander from side to side, will naturally erode material from the outside of the bend and deposit it on the inside of the bend. This creates different flow rates and different habitat niches, which in turn support different wildlife — for example a wader such as a sandpiper will use the shingle banks on the inside of the bend; a steep bank on the outside of a bend is a prime spot for sand martins to burrow for their nests.

Rivers naturally wiggle up and down, so there are shallow riffles and deep pools which also provide different habitat. A shallow riffle with a stony bed is the place to find salmon parr who are well adapted to living in fast flowing water and hiding from predators, and each other, using stones. The slower, deeper pools will hold bigger trout who don’t expend any more energy than they need to, and the deep pools stay cooler during hot summers.

Much of the practical project work that the WTT delivers involves various techniques to put the wiggles back, which can be anything from a tree in the river to kick the flow from side to side and scour a pool, to using large diggers to reshape the river channel and bed.

The video above describes creating meanders and variations in depth in the River Went in Yorkshire.

The video below is from the River Meon. WTT Conservation Officer Andy Thomas describes a variety of techniques including use of a large machine to create meanders and variations in depth — deeper pools and shallower riffles. 

This question is posed in this way because barriers such as weirs, culverts, sluices and hatches are so common that it is remarkable to find rivers without them, particularly in England. Research has shown that there is a barrier ever 1.5km in the UK, and only 1% of our rivers are free of man made barriers. Many of these are historical and associated with industry, flour or fulling (fleece washing) mills and water meadow management and no longer have a function in terms of generating power or managing water but are of heritage interest. Others are currently in use — for example, gauging weirs used to record river levels for flood management. The vast majority of weirs, sluices and hatches no longer have a function. 
Culverts can be a more intractable problem as they are used very widely under roads and railways, sometimes as low cost cheap alternative to a clear span bridge or, in urban areas, to bury a river and create building land.

The image on the right below shows how culverts can be impassable for trout migrating upstream to spawn.

As well as fragmenting river habitat and acting as a barrier to up and downstream migration for trout and other fish, barriers disrupt the way that rivers transport sediment. Above the barrier, the river is impounded and flow rates reduce, leading to sediment being deposited. Below the barrier, the river compensates for the loss of sediment by increasing rates of erosion downstream.
The video below uses the ​‘EMRIVER’ modelling to to demonstrate the effect of weirs on river function.

To return both habitat connectivity and natural sediment processes, the best thing to do is remove the barrier completely. Where this isn’t possible then creating a by pass channel around the barrier, or reducing the size of the barrier is the next best option. If it a small barrier, reducing the height or cutting a notch will help. 
Building an engineered fish pass is usually expensive and may resolve the fish migration issues, but they are expensive to maintain and they don’t tackle the root cause of the problem. 

Culverts can be replaced by clear span bridges in some places and in urban areas, the ​‘daylighting’ of previously buried river channels is an expensive but very effective option as it will create blue/​green space and can be used for flood storage. See examples of the Porter Brook pocket park in Sheffield and the River Quaggy in London.

Rivers will naturally erode the river banks and river bed, and then deposit that material further downstream and eventually out to sea. Rivers on steep gradients have more energy to carry out erosion than lowland rivers or chalk streams so the rate of erosion is higher on steep gradients. Geology and climate (catchment scale factors) also have a role to play — relatively soft rock such as sandstone will erode more readily than granite, and high river levels gives the river more erosive power. 

Erosion and deposition by rivers can be considered problematic. In the UK, we like to ​‘manage’ the natural environment based on our priorities, whether that is preserving a bank where sand martins nest, not losing gazing land in a field or removing gravel that has built up against a bridge. Preventing natural river processes can take a lot of engineering in high energy rivers and is doomed to either fail or need constant maintenance.

What we look for on our Advisory Visits is artificially high levels of erosion that can be resolved by working with natural processes. These are two examples that we see most frequently in rural areas: 

• livestock (especially cattle) breaking down and poaching river banks, leading to an over-wide, shallow channel that warms up quickly in summer and has excessive amounts of silt spread across the river bed.

• grazing of livestock (mainly sheep) up to the edge of the bank, so that only the very shallow roots of grass hold the bank together rather than deeper rooted plants and trees. This results in chunks of the bank breaking off into the river — a higher than natural level of sediment in the river that can smother spawning gravels and river plants.. 

The root cause of these issues is livestock management, and the simplest and most effective answer is to have an ungrazed or very lightly grazed buffer strip along the river bank. Over time, the river will return to a natural level of erosion and adjust the channel width through natural processes.

Left to right: A cattle drink on the River Haddeo in Devon results with sediment entering the river from the cattle poached area; cattle have broken down the bank on the River Itchen resulting in an overwide channel; A steep eroded bank, grazed by sheep. 

The root cause of these issues is livestock management, and the simplest and most effective answer is to have an ungrazed or very lightly grazed buffer strip along the river bank. Over time, the river will return to a natural level of erosion and adjust the channel width through natural processes.
To speed things up, especially in low energy rivers, it is also possible to deflect the flow against an eroding bank using a felled tree, or protect the bank using woody debris or willow spiling backfilled with hazel faggots which will capture sediment and establish protective plants and shrubs. Rivers can also be made narrower using these techniques or by using machinery to re-profile the river banks.

With climate change and increasingly extreme weather leading to floods and droughts, naturally functioning rivers and their floodplains should be seen as a huge asset. 

If rivers have ​‘roughness’ (rocks, woody debris in the channel and trees on the banks, and if they are connected to their floodplains and meander, they will slow the flow of flood water. Slowing the flow is key component of natural flood management.

A straight, smooth dredged channel may be designed to ​‘get the water away’ but it simply speeds the flow of water to cause an (even more) catastrophic flood when the water hits an inevitable pinch point such as a road bridge or culvert.
To cope with droughts, rivers need to have a ​‘low flow channel’ so that the water in the river is concentrated into a smaller area, reducing the loss of water by evaporation and allowing greater depth which helps water to stay cool. Deep pools and shade are also crucial for life in the river to survive in a drought.