Streams and rivers, symbols of constant change, are in reality extremely ancient and surprisingly stable in form. In many cases they serve the same drainage function today as when created by great earth uplifts millions of years ago. However, while eroding the land to its present profile some stronger rivers have cut into and captured the headwaters of weaker streams. During the recent Ice Ages, glaciers obscured or in a few cases completely altered old patterns, for example by diverting the Severn southward away from its old course into the River Dee. But even here much earlier patterns are often discernible.
The character of streams and rivers, like that of springs, ponds and lakes, is affected by the geological and climatic division of Britain into two main areas: a wet, rugged north and west and a relatively dry and gentle south and east. More local differences are also important. In the Scottish Highlands, westward-running rivers are short and torrential, plunging in a few miles from mountain to sea, often through a series of lochs or lochans. Those flowing east have a longer, more gradual course and so exhibit more of the ‘typical’ river features described below. The same distinction between westward and eastward-flowing rivers holds, in general, throughout the uplands, although some Pennine streams, such as the Tees, change character repeatedly where they descend in a series of steps.
The ‘typical’ river originates in uplands where it actively erodes, bringing down great quantities of debris. The gradient of the river bed lessens as the river flows through the foothills to the lowlands, and as the flow becomes slower erosion decreases and deposition increases. The river slowly winds across its flood plain, the banks being raised by deposition until the river may be higher than the surrounding land. During flood the banks of the river may be breached and the course of the river changed. The processes of erosion and deposition result in the river developing a smooth gradient from source to mouth, until geological upheaval or change in sea level causes the river to be rejuvenated, with renewed erosion and deposition.
Wherever the surface material is chalk or limestone, watercourses often exhibit peculiarities which set them apart from streams and rivers elsewhere. Whether in the Wealden Downs, Wiltshire or the limestone regions of Derbyshire and North Yorkshire, many streams are ‘winterbournes’ which flow only seasonally. Even if permanent, limestone streams are likely to change level dramatically in response to seasonal changes in rainfall. Dry valleys are common, indicating now-vanished waterways which once were active. Rain water, containing dissolved carbon dioxide from the air, acts as a weak acid and slowly dissolves the limestone itself. Joints and cracks within the limestone become enlarged until the stream disappears entirely down a ‘swallow hole’, and flows through underground passages and waterfalls to emerge some distance away. Caverns are slowly formed until eventually the roof may collapse leaving a narrow gorge, such as Cheddar or Goredale.
In the levels of Somerset and East Anglia, rivers have usually been modified beyond recognition. In their natural state, these watercourses flowed sluggishly, in sinuous meanders or multiple channels through marsh and fenland, for nearly their whole length. The ground water table was so high along much of their courses that any major rainfall caused widespread flooding, important for the development of the wetlands communities on either side.