You will learn of the different sources that create a river, the paths that it takes and the effects it has on its surroundings as it travels along the way.
Mountains, Rivers and the Water Cycle
Most rivers begin high up on hills and mountains. If you’ve been walking in hills, you’ll know they can be surprisingly damp. That’s because the ground acts like a sponge soaking up rain.
Excess water flows downhill, pulled by gravity, first in trickles and then small streams called rivulets, which in turn join together to form larger streams. Finally, a stream becomes a river. We’re going to follow a river from start to finish: the River Forth, in central Scotland.
The place where a river starts is called the source. Typically, sources will be found on high ground, on hills or mountains where the slope is steep. Rainfall and melting snow provide the water to feed rivers and lakes.
The lake (or loch, as it is referred to in Scotland) shows the water cycle in miniature. As they pass over hills, clouds empty rainfall, which flows downhill to fill the loch.
Evaporation, Condensation, Rain
Heat from the Sun evaporates water from the loch surface. It rises into the air, condensing in the cooler temperature to form clouds. The clouds become denser with moisture until the cycle starts again with rainfall.
The Upper Course
As a river tumbles down from high ground, it cuts through the earth. This part of a river is called the upper course. The rushing water erodes the landscape in different ways according to the kind of ground it passes over.
This vertical erosion creates steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges. The water carries small parts of rock and earth eroded by the river, which then become part of the river, and contribute to erosion and deposition downstream.
When hard rock overlies soft rock, water passing over erodes the soft rock more easily. This creates steep ledges in the rock bed over which water cascades. Waterfalls can be spectacular, reaching great heights.
Over time, the soft rock beneath a waterfall continues to erode. This creates an overhang of the harder rock. Eventually, the hard shelf collapses under the water’s weight, and the waterfall moves further upstream. The resulting valleys are called gorges.
In the upper course, water is pulled downwards by gravity. This can be seen most dramatically in waterfalls. Later, as the river levels off, the water is directed horizontally.
Loch Ard – Source of the Forth
There are 22 lochs in the Loch Lomond & the Trossachs National Park in Scotland, constantly replenished with fresh water pouring down from Ben Lomond and the other mountains in the area. Loch Ard collects water from the surrounding hills and then feeds into the River Forth.
A small outlet on the loch’s eastern side continues to flow eastwards, growing wider and fed by smaller rivers, across to Scotland’s east coast and into the North Sea.
Loch Ard is an “open” lake, which means that water flows constantly out to supply the River Forth. Open lakes are found in areas where water gained from rainfall exceeds that lost by evaporation.
Carved by Glaciers
Scotland has an abundance of mountains and lochs. Many of its geographical features are a result of it having once been covered by glaciers, which eroded and shaped the landscape, creating the dips that fill up with water because of the region’s high rainfall and low evaporation.
Loch Ard is a freshwater lake, fed by rainwater and streams coming down from surrounding hills. In contrast to seawater, which is salty, freshwater can be treated for human consumption. Different kinds of fish can live in freshwater habitats than in saltwater habitats.
The Middle Course
The river is wider and deeper in the middle course. The water appears less turbulent than in the upper course and its gradient is gentler, but it is actually moving faster. The increased velocity and volume of the water gives it greater force to erode riverbanks.
Sediment – small pieces of rock and earth – is carried along with the river, increasing its erosive power, and deposited as the river winds to create distinctive river features.
This bend is gentle but, over time, river processes will make it more pronounced. The river is faster and deeper on the outside of bends, eroding the bank. Meanwhile, sediment is deposited by slower-moving water on the inside of bends.
Along its course, the main river is built up as many smaller streams and rivers join it. These smaller channels are called tributaries. The point where one river joins another is called a confluence.
Historically, humans have often chosen to live near rivers, which provide water for drinking, washing, transport, fishing and recreation. However, rivers are obstacles. To get across rivers, we build bridges, which can be spectacular and beautiful constructions.
This is the view from the Wallace Monument on the summit of Abbey Craig, near Stirling. The monument commemorates Sir William Wallace, a 13th-century Scottish hero.
From here, you can see the middle course of the Forth as it winds its way across the countryside. This bending path is typical of the middle course of a river.
The path of a river twists and turns around stones and other obstacles and is affected by gradients of the land. Some parts of the river flow faster than others. These things mean that a river follows a winding course – it meanders. The meanders become more pronounced over time.
The middle course sees continual changes. The volume of water passing along the river produces effects such as erosion (wearing away of banks), transportation (movement of eroded soil and other material), deposition (material being deposited) and flooding (overflowing).
Deposition happens where a river does not have enough energy to carry material further and deposits it instead. This happens in the middle course of a river, or on the inside of bends or meanders.
Flooding occurs anywhere in the middle/lower courses of a river, especially where land is flat and the river carries a high volume of water. When the river gets too full, it can overflow, with water spreading across the floodplain.
Firth of Forth
The Firth of Forth is where the river flows out to the North Sea. “Firth” means an estuary, which is where a river meets the sea. Estuaries are often home to many different forms of wildlife. Around a third of people in the UK live close to estuaries.
Estuaries are much wider than rivers inland. The Forth Bridge, built in the 19th century, is almost 2500m across and allows transportation to cross the wide estuary.
Estuaries are typically found in areas of wide, flat land. Deposition continues to occur at an estuary, but the sediment it leaves is swept away at high tide.
To the Sea
At estuaries, freshwater from rivers meets salty seawater, producing a mixture called brackish water or briny water. The mixing circulates sea nutrients and expels waste products, creating rich, dynamic ecosystems.
Estuaries provide rich and varied habitats, especially for birds. With the diverse plant, fish and other animal life, there is a lot for birds to eat. Migratory birds use estuaries as useful resting stops on their travels.