River Profiles and Downstream Changes

Following the River Tees from source to mouth

A river landscape includes both the water-filled channel and the valley around it. These change together as a river travels downstream, but they are not the same feature: a channel can be narrow within a much wider valley.

The River Tees, in north-east England, provides a named example. It begins at Cross Fell in the Pennines, 754 m above sea level, and flows through upland and lowland landscapes to the North Sea at Teesmouth. Its upper, middle and lower courses show how channel shape, valley profile, gradient, flow and sediment are connected.

Two ways to view a river landscape

A long profile is a side view of the river from source to mouth. It shows how height and gradient change. A typical long profile is concave: steep near the source, then becoming progressively gentler towards the mouth. On the Tees, the steep upland course gives way to a gentler middle course and an almost flat lower course.

A cross profile cuts across the river at right angles to its flow. A channel cross profile shows the width and depth of the water-filled channel. A valley cross profile includes the surrounding slopes and valley floor. Comparing these views helps explain why a narrow upland river looks so different from a broad lowland river.

The upper course: a narrow channel in a steep valley

Near Cross Fell, the Tees has a narrow, shallow channel, a steep gradient and a rough, uneven bed. Its sediment includes large, angular pieces of rock supplied by weathering and movement of material from the valley sides. These particles have not yet travelled far enough to become heavily worn down.

Vertical erosion cuts down into the river bed. Weathering and mass movement also break down the valley sides and move material downhill. Together, these processes produce a narrow, steep-sided V-shaped valley.

The river winds around projecting ridges of higher land called interlocking spurs. These occur on the upper Tees near Cauldron Snout. Resistant whinstone also produces a rough bed and rapids between High Force and Low Force.

Discharge is relatively low because only a small area supplies water near the source and few tributaries have joined. Although the gradient is steep, the shallow water encounters considerable friction from the bed and banks. Average velocity is therefore generally lower than farther downstream, even though individual rapids can be fast-flowing.

The middle course: a wider valley and developing meanders

Around Barnard Castle, the Tees has a gentler gradient and a wider, deeper channel. Tributaries add water as the river travels downstream, increasing its discharge: the volume of water passing a point each second.

The larger flow gives the river more energy for erosion and transport. Erosion enlarges the channel, while lateral erosion becomes increasingly important. This sideways erosion helps widen the valley and develop meanders, or bends, in the channel. Valley sides become gentler and floodplains develop beside the river.

Within a meander, erosion on the outer bend and deposition on the inner bend help the bend grow and shift sideways. This contributes to the widening of the valley floor over time.

Sediment is now generally smaller, smoother and more rounded. As particles collide during transport, attrition chips away their corners and breaks them into smaller pieces. Larger material is also deposited sooner, while finer particles can travel farther downstream.

The lower course: a broad floodplain and large channel

Near Yarm and Stockton-on-Tees, the river crosses much flatter lowland terrain. Its gradient is very gentle, and its channel is generally at its widest and deepest. The valley has a broad, flat floodplain rather than the steep-sided profile found near the source.

The Tees has large meanders between Darlington and Yarm; Yarm itself lies within a meander. Oxbow lakes occur in parts of the lower course where former bends have been cut off. Deposition contributes to extensive floodplains and natural levees beside the channel.

Discharge is generally greatest here because water from the tributary network has accumulated. Fine sediment, such as sand and silt, can be carried much farther than large upper-course stones, including in suspension within the flowing water.

Deposition is important, but it does not mean that the river has stopped eroding or transporting sediment. Different processes can occur in different places along the same stretch of river.

Why velocity can increase as gradient decreases

Velocity means the speed of the water, whereas discharge measures its volume per second. A river can have a large discharge without every part of its flow being equally fast.

A steep gradient encourages faster flow, but gradient is not the only control. Downstream, the channel becomes wider and deeper, so a smaller proportion of the water is in contact with the bed and banks. The bed also generally becomes smoother as coarse, angular material gives way to finer sediment. Friction therefore has less influence on the flow.

These changes, together with increasing discharge, explain why average velocity generally increases downstream despite the decreasing gradient. The overall pattern is a change from a small, rough upland channel to a larger, more efficient lowland channel. Local rock outcrops, bends and rapids mean that the pattern is a trend, not an identical change at every point.