Investigating Rivers Using Maps and GIS

Reading the river and the surrounding landscape

A river map gives two connected kinds of evidence. The shape of the channel shows bends and disconnected water bodies, while the surrounding relief shows the valley through which the river flows. Reading both together is more reliable than identifying a feature from one clue.

Ordnance Survey (OS) maps use contours to show height above sea level. Closely spaced contours indicate steep slopes; widely spaced contours indicate gentle slopes. Read the contour heights as well as their spacing: gently sloping land can be high above sea level.

The two map scales used here are 1:25,000 and 1:50,000. At 1:25,000, 1 cm on the map represents 250 m on the ground. At 1:50,000, it represents 500 m. The 1:25,000 map shows the same ground distance at twice the paper length, allowing closer examination of small features. Always use the map’s own key and contour interval.

Recognising river landforms on OS maps

A V-shaped valley has steep sides and a narrow valley floor. Look for closely spaced contours on both sides of the river. Where contours cross a valley, their V-shaped pattern generally points upstream. A valley cross-section helps confirm whether the sides descend steeply towards a narrow floor.

Meanders appear as bends in the channel. An oxbow lake is a curved water body separated from the main river, often beside a meander. Its detached position matters: a bend still connected to the channel is not an oxbow lake.

A floodplain appears as a broad, gently sloping valley floor beside the river, with relatively few contours across it. Read this alongside the channel shape: meanders within a broad, flat valley provide stronger evidence than sparse contours alone.

A waterfall may be named or marked on the map. Closely spaced contours and a cliff near the river can suggest a sudden drop, but these clues alone do not confirm a waterfall. Use the map key and any label to strengthen the identification.

To investigate valley shape more closely, choose a line across the valley and place a strip of paper along it. Mark each contour crossing and its height, together with the river’s position. Transfer these positions to a graph with distance on the horizontal axis and height on the vertical axis, then join the plotted heights. This is a cross-section across the valley, not a long profile along the river.

Linking a long profile to BGS geology

A long profile plots river height against distance downstream. Rivers commonly have a steep upper course and a progressively gentler gradient towards the mouth, but local changes in gradient can interrupt this overall pattern.

Geology helps explain such variations. Rocks differ in their resistance to erosion. A resistant section can help maintain a steeper reach or a step, while neighbouring less-resistant material is eroded more readily. Waterfalls and rapids can therefore be associated with changes in rock resistance. However, a rock name or map colour alone does not establish how resistant that rock is.

British Geological Survey (BGS) maps distinguish bedrock from superficial deposits above it. This distinction matters in a river valley: river-deposited sediment at the surface is not necessarily the rock controlling the deeper valley structure. The BGS Geology Viewer allows you to inspect both types of geology, use different base maps and display geology over a three-dimensional terrain model.

To connect geology with a long profile:

1. Trace the river from source towards mouth and establish height at selected points using mapped height information.
2. Measure distance along the channel and plot height against distance downstream.
3. Follow the same river route on the BGS map. Use its legend and location information to identify the geological units crossed.
4. Mark where geological boundaries occur along the profile, using distance downstream to align the two sets of information.
5. Compare changes in gradient with changes in geology, then explain the relationship using supported information about rock resistance.

For example, suppose a river falls 80 m over one 2 km reach and 20 m over the next 2 km reach. The first reach is steeper because the fall is greater over the same distance. If the change between the reaches also coincides with a boundary between rocks of different resistance, geology offers a possible explanation. This is a constructed demonstration, not a measured river profile.

Transfer this method to a real river by matching actual channel positions, height measurements and BGS geological boundaries. The BGS viewer keeps geological layers visible when zoomed in, but zooming does not create finer geological detail: its underlying data have a limited resolution.

Investigating human intervention with maps and GIS

Human intervention means deliberate alteration of a river landscape. An investigation needs a clear question, such as: has a mapped alteration changed the length or shape of this river reach? Maps can reveal the change; further evidence is needed to establish who caused it and why.

A GIS stores geographical information in layers. Displaying geology over a base map is one example. For a river investigation, dated channel information, habitat surveys and imagery can be compared over the same area. OS maps at 1:25,000 and 1:50,000 allow you to examine channel shape and the surrounding landscape; more detailed survey layers can show changes too small to identify on these maps.

Begin by locating the same reach on each map or layer, using fixed reference points such as the ends of the study area. Check dates and scales before comparing. Record visible differences in channel shape, disconnected bends and nearby mapped structures. Measure the same feature in the same way on each version; a winding channel must be measured along its bends.

Consider a fictional comparison in which the channel between two fixed points measures 1.2 km on an earlier map and 0.8 km on a later map. The mapped channel has shortened by 0.4 km, or 400 m. If the earlier winding route has been replaced by a straighter route, this supports a conclusion about changed channel length and shape. Documentary evidence of engineering would be needed to attribute the alteration confidently to human intervention rather than natural change.

A real GIS investigation: Seven Hatches on the River Wylye

At Seven Hatches, just upstream of Wilton, the River Wylye had been widened and deepened. Sluices obstructed fish migration and held back flows upstream. Restoration within the STREAM project included adding gravel to the bed, creating riffles, narrowing parts of the channel and introducing woody debris. These measures aimed to create more varied channel conditions and improve habitats.

Investigators surveyed conditions before and after restoration and used a control site with comparable physical characteristics. Field mapping was converted into GIS layers so that habitat areas could be calculated and compared through repeat surveys. The GIS also recorded physical and ecological features and the locations of samples and cross-sections.

To use this approach, first locate the restored reach on an OS map and define its boundaries. Then align dated survey layers in GIS. Represent each mapped habitat area as a closed shape, or polygon, give it a consistent habitat label and calculate its area. Comparing the area of the same habitat category before and after restoration shows whether its mapped extent increased or decreased. Apply the same categories and measurements to the control reach. This moves the investigation beyond simply noticing that a river looks different.

At Seven Hatches, added gravels and riffles helped create more varied flow types. Woody debris also increased local flow variation, with evidence of scour downstream of the structures. However, changes in channel width, depth and velocity occurred alongside natural variations in water levels and catchment processes. Some vegetation changes were not significant compared with those at the control reach. The evidence therefore supported physical changes without proving that every observed ecological change resulted from restoration.

A useful conclusion separates observation, interpretation and limitation. Maps and GIS can locate alterations and measure changes in channel shape or habitat area. Engineering records help establish the intervention’s cause, while repeated field measurements and a control site help assess its effects. Channel shape alone does not demonstrate a reduction in flood risk or an improvement in water quality.