The River Chess catchment and beyond
In this Storymap (2) we examine factors affecting the River Chess that come from outside the catchment (land use, transport links, climate change) and those that occur on a catchment scale (such as geology and hydrology). In Storymap 3 we focus in on water quality and ecological issues that affect different stretches (reaches) of the river.
Introduction
Land use change in the Chilterns AONB
From 2006 to 2010 the Chiltern Conservation Board carried out a land use survey in the Chilterns Area of Outstanding Natural Beauty. A selection of 1km squares was surveyed in detail to generate the data. They found that 34% of the area surveyed was cropped arable land, 24% was agricultural grassland and 19% was woodland and scrub. At that time the area of land under crop (arable) was rising. Ploughing land to grow crops can enhance soil erosion and move sediment and pollutants into rivers. For example, soil and fertiliser can wash from fields into rivers during heavy rainstorms. Farmers can help protect rivers such as the River Chess by following good agricultural practice guidelines.
Here you can find More detail on land use and land use change with time from the Chilterns Conservation Board.
Urban areas have spread widely over the last few decades, explore by dragging the slide bar in Figure 1 to see changes in Chesham and elsewhere in the catchment. The spread of urban areas has:
- Increased abstraction pressures on the chalk below the Chess for drinking water supply (see Hydrology section by following the link at the top of the page)
- Increased runoff (by increasing impermeable, paved areas) and increased the amount of sewage that needs to be processed each day.
Figure 1] Drag the Slider to show the two maps: landuse 1940-47 (left) and present day (right). Urban areas are shown as black rectangles (buildings) in the old map and as greyish areas in the new map. Urban areas have expanded dramatically between the two dates.
Understanding the River Chess at the catchment scale
A river catchment is the area of land surrounding a river in which any rainfall will ultimately drain into the river. The map below shows the catchment of the River Chess.
This next section of our second Storymap includes a discussion of the hydrogeology (the way the groundwater system works); the hydrology of the river and water abstraction. Water quality issues in the River Chess are discussed in Storymap 3.
Hydrogeology
Basic chalk stream system:
Water in a chalk stream is generally groundwater fed. By this we mean that some of the water that falls as rain onto the ground seeps through the soil into the chalk, and contributes to the aquifer. The aquifer is an underground layer of porous rock that can store water (called groundwater) and transmit water through its mass. The water table (shown in this diagram by a dotted line) denotes the top of the aquifer, and varies over time. Water from the aquifer can flow into the stream. It can also leave the catchment as groundwater outflow or be abstracted for use from boreholes.
Figure 6] Interactive graphic showing gaining and losing rivers, drag the arrows to see the difference. This is a simplified graphic which applies when the aquifer is 'simple' such as sand. The chalk aquifer, as we go onto explain, is more complex.
Figure 7] Bands of flints and marl in the Chalk along which water flows in fractures and pipes. Photo by Andrew Farrant. Reproduced with the permission of the British Geological Survey ©UKRI [2021]. All rights reserved.
Stretches of chalk stream can be losing or gaining in character. 'Gaining' stretches of the river are receiving (or gaining) water from the aquifer, whereas in a 'losing' reach river water is lost to the aquifer. Chalk streams are dynamic systems and water flows and movement depends on the time of year. A stretch of chalk stream can switch from being gaining to losing (or indeed losing to gaining) depending on the groundwater level relative to the riverbed in an unconfined aquifer. When the water table drops below the riverbed a stretch of river can start to lose water. The same stretch can start to gain water once the water table has risen again.
The chalk aquifer has fractures and fracture sets (Figure 7) which dramatically increasing its permeability (the ease and speed at which water moves through the aquifer). Water moves more rapidly through the fractures in the chalk than it does through the chalk matrix.
The flow in the river in an unconfined setting is strongly linked to changes in groundwater level. This relationship is shown in the scatter plot below which plots the level of groundwater in mAOD on the horizontal x axis against river flow (vertical y axis). Each dot represents one time in the river's history. Clearly the higher the water table, the more the flow.
Figure 8] Groundwater level vs river flow for the Chess.
Springs
The River Chess has both springs and artesian boreholes. A spring is a point on the land surface at which water flows from the ground. The water is often very clear as it has been filtered by the aquifer, and it will contain minerals from the rocks that it has passed through on its journey to the surface.
Figure 9] Springs, fractures sets and the water table in the Chess catchment. Note that the shape of the fracture set is not how it would actually occur in nature, however, it is close enough to reality to explain the concept.
As noted above, groundwater flow in the chalk aquifer is mainly through fractures, these are cracks or breaks in the chalk through which water can flow. Water does flow through unfractured chalk but it happens very slowly. A spring usually occurs where a fracture set (or single fracture), with a source of ground water (for example, the water in the upper part of the set in the left of the figure), intersects with the ground surface (Figure 9). The unfractured chalk above and below the fracture set acts as a confining layer, channelling the water along the fracture set.
An artesian borehole or artesian well is similar to a spring, in that water flows out of the ground without pumping (Figure 10). However, the borehole or well needs to have reached an aquifer from which the water flows. How artesian boreholes operate in the chalk of the Chess valley is explained in detail below.
Figure 10] Water flowing out of the top of an artesian borehole in the Chess
Artesian Boreholes
Most boreholes require pumping to produce water, the water does not flow out of them of its own accord. Whereas, artesian boreholes flow without pumping, and they occur under special conditions which are shown in Figure 12.
Figure 12] A cartoon explaining how boreholes drilled into the fracture set in the chalk can be artesian. As with Figure 9, the shape of the fracture set is not realistic but the relationship between water table up the valley and the confining chalk around the fracture set is realistic and explains the basic system.
As with the spring figure (Figure 9), Figure 12 shows rainfall infiltrating into the ground high up in the valley into a fracture set (left). The water table in this area is above the ground level lower down the valley, this is key to producing artesian borehole flow. Lower down the valley, in the fracture set, the water is trapped above and below by the confining chalk. It is under pressure because the water in the fracture set higher up the valley (left) is pressing down on it. If boreholes are drilled into the facture set in the locations shown they provide a pipe along which the pressurised water can flow. If the pressure is enough to reach the surface, the borehole is artesian.
Flow from springs and artesian boreholes may dry up completely or increase dramatically depending on the groundwater level. The plot below shows two groundwater monitoring boreholes at two different locations and depths in the aquifer to illustrate the variability in groundwater level at two different depths; over a range of 20m at Ashley Green (a deep borehole) and over 6m at Wayside (a shallow borehole) over a 20 year period. You can see that the variations in height of water in the two boreholes is very similar over time.
Figure 13] Water levels in two boreholes in the Chess 2000-2020. Note that the y axis is split into two and that the plots are at different scales.
Figure 14] A cartoon illustrating how the chalk beneath the Chess may actually operate as a dual or multi-aquifer system.
Chess Multi-Aquifer System?
We have explored the concept that the aquifer beneath the Chess is not a homogeneous mass of chalk. Much of the flow in a chalk aquifer actually occurs along numerous fractures (see Figure 7 and 14), as well as more slowly through the connected small pores in the chalk. Different fractures or sets of fractures can be separated from each other by confining chalk, (Figure 14) producing, in effect, a dual or multi-aquifer system.
Hydrology
The flow of water in the Chess is intimately linked to the hydrogeology as we explored in the previous section, and the source(s) of the river are controlled by position of the water table. In this section we explore the concept of flow over time: how does the river's character affect flooding and droughts?
Disappearing river
The source of the Chess varies in location. It tracks the point where the water table reaches the ground, so the seasonal variations in water table height alters where the river starts from. During the winter the source of the river will be further up the valley, during summer, as the water table drops, the source migrates down the valley. This gives rise to the term 'winterbourne' for chalk streams, literally meaning a 'winter river'. The winterbourne section is ephemeral, meaning it does not flow all year round (Use the slider in Figure 15 to see this illustrated). Over recent years, lower-than-average rainfall has reduced recharge resulting in lower groundwater levels. This has caused flow in the Chess to disappear from river reaches in Chesham.
Figure 15] Cartoon showing different water tables in winter and summer which cause Winterbourne behaviour. Drag the slider to see the winterbourne section of the Chess appear and disappear. Note that for ease of understanding, this is a simplified representation of the chalk aquifer which does not show fractures or fracture sets.
In the animation below (Figure 16) we have mapped the approximate extent of the Chess on particular dates over a period of years. On some dates, you will see that the river appears, disappears and then re-appears. This is a real representation of the flow in the river rather than a mistake.
The 'perennial' section of the river flows all year round, and the 'perennial head' is the highest point in the valley where there is continuous flow all year round . The perennial head of the Chess is at Lords Mill. Here water flows out of a mystery pipe that could be linked to an artesian borehole.
Figure 17a] A postcard of Lords Mill at Chesham past (left) and present (right)
Figure 17b] Strong, clear water flow from the 'mystery pipe' at Lords Mill
Patterns in flow over time:
Flows in the River Chess fluctuate over time in response to natural changes in rainfall and temperature. Here we examine some of the reasons for these variations in flow.
The River Chess, like many chalk streams, will have a natural annual cycle illustrated here using data from the Environment Agency from 2000 to 2020. As you can see river flows increase through winter and this is due to groundwater rise in shallow sections of the aquifer in response to autumn rains. The rise in flow continues until spring when warmer temperatures, combined with lower rainfall causes groundwater levels to decline. The flow decreases over summer and autumn as groundwater levels drop.
Figure 18] Mean daily river flow over 20 years (Rickmansworth gauging station, cumecs = cubic meters per second). Contains public sector information licensed under the Open Government Licence v3.0 .
Year-to-year variations in flow also occur because some years are wetter than others. In fact some scientists now think that 7-year cycles in annual rainfall totals and groundwater level linked to the North Atlantic Oscillation may lead to changes in flows in chalk rivers. The River Chess experienced marked drought conditions in 2005/6 and 2018/9 which we highlight below.
Measuring the flow
A hydrograph is an illustration of water flow with time for a given point on the river. Hydrologists measure water level and water flow in rivers to help with water resources planning, and for flood warning purposes. The Environment Agency monitor water levels in the River Chess with fixed equipment at three locations; two in Chesham and one in Rickmansworth.
Figure 20] River Chess stageboard and gauging stations (Purple Arrows)
Figure 21] Stageboard at Chesham (November 2020)
By clicking on the 'Real Time Water Flow Dashboard' below you can see data for the River Chess provided by the Environment Agency monitoring stations (Rainfall from Chenies, the stage board at Chesham and the water level at Rickmansworth). We update these plots daily so that you can see up-to-date information.
For scientists trying to understand the hydrology of rivers it helps to have more information on water level and flow intermittence throughout the river catchment. This is where you can help: The CrowdWater app allows you to submit water levels at one or more locations at different times, it even has a feature which adds a virtual stage board (a board marked with height to measure water level) to a location. If you walk along the River Chess we would encourage you to download and use this app to add valuable data to our understanding of the water levels in the Chess River. Find out more about this app and how you can help in Storymap 4.
Ecological Flow
To be a healthy environment, a river must achieve a minimum flow: below this minimum flora and fauna will not flourish. This gives rise to the concept of ecological flow which defines not just the minimum flow needed but also minimum values of water quality required to keep the river environment healthy. Prolonged drought conditions can cause problems for plants and animals.
One piece of infrastructure that is helping to maintain higher flows in the River Chess downstream of Chesham is the sewage treatment works. The flow from the sewage treatment works comprises 60% +/- 14% of the total flow in the upper sections of the Chess above Latimer (based on monthly flow data from 2015 to 2020). This puts treated water, of a different chemical quality compared to groundwater, into the river. You can learn more about this in Storymap 3.
Figure 22] River Chess Sewage Treatment Works
Abstraction of water
Abstraction of groundwater for public water supply can have an important effect on groundwater levels in many chalk catchments. Affinity Water and Thames Water are the local water companies that abstract groundwater from the Chilterns for drinking water supply. The three locations of water supply boreholes are shown in Figure 23. The River Chess catchment has been investigated to assess the impact of abstraction on river flows and ecology, and designated as over-abstracted for some time. In September 2020, Affinity Water stopped abstracting water from two sources around Chesham. The company previously removed a maximum of 9 Mega-litres per day from these groundwater sources. Thames Water currently abstract c. 6-8 Mega-litres per day from groundwater in Chesham, and supply drinking water to Tring which is discharged to the River Thame. There are plans to stop this abstraction here 2025. There is one further source owned by Affinity Water in the lower catchment, which supplies water to the local area.
Figure 23] The location of three public water supply boreholes in the Chess catchment. The exact locations are not shown, the borehole are somewhere within the blue circles (not at the circles centre).
As we explained earlier, in the River Chess it is not clear how strong the link between abstraction and water level is because of the complexity of the chalk aquifer in the catchment. Reductions in groundwater abstraction could mean increased river flows over the coming years, and this is something the River Chess Association and Chiltern Chalk Streams Project would like your help to assess. If you walk along the River Chess, or live nearby, you can help by taking photos of the river during your walks. If you are interested in helping out visit Storymap 4 to find out how.
The final part of the story is how much water people use in the catchment. Water consumption in the catchment per capita is very high. The average person who lives in the Chilterns area uses 176 litres of water per day compared to a UK average of 148 litres per day. But did you know that there is less rain falling in London each year than falls in Rome (Italy)? We can all play our part to conserve this precious resource, getwaterfit is an app that can help save water useage in our own homes.
Figure 24] Proportion of average household water usage by activity (SOURCE: Affinity Water).
Figure 25] Road runoff at Latimer Park arising from intense rainfall
Climate Change
In the future lower summer rainfall and hotter temperatures may mean that less water reaches the aquifer and river flows in summer are reduced. In addition, if rain falls in more intense bursts (also predicted to occur due to climate change) then overland flow to the river occurs at the expense of rain infiltrating the soil and recharging the aquifer. This effect, which may reduce the recharge of the chalk aquifer in summer, is sometimes referred to as ‘the wrong type of rain’. Factors affecting seasonal recharge of the aquifer (contrasting summer and winter) are illustrated in Figure 26a and 26b below.
Figure 26a] Cartoon showing how much rain reaches the chalk aquifer in summer
Figure 26b] Cartoon showing how much rain reaches the chalk aquifer in winter
User Survey
ArcGIS Survey123
Attribution
This Storymap was created by researchers at Queen Mary University of London (QMUL) with funding awarded by the Natural Environment Research Council (NERC) as part of UK Research and Innovation’s (UKRI) rapid response call to COVID-19. Support in the form of photographs, videos, data and expertise was provided by the Chilterns Chalk Streams Project, River Chess Association, Environment Agency, Thames Water, Affinity Water and ESRI