When the Water Stops Moving: What Happens in a Drought Pool?

At our New Forest Water Watch stall today, we used a simple experiment to demonstrate something that is normally invisible: oxygen in water can be used up. As microorganisms respire and consume oxygen, the blue colour disappears. It is a simple demonstration of an important process, and one of particular relevance to what is happening in some of our New Forest streams after the drought.

The upper reaches of Highland Water still contain what the National Park describes as “isolated pools and gravel bars”: great banks of shingle that divide the channel into separate pools. These pools are gradually joining up again, but very slowly. Some are connected by tiny trickles; others remain completely separated by dry gravel. It is tempting to think simply about the water disappearing, but what happens inside those remaining pools is equally important.

A pool can be a refuge and a trap. When a stream like this becomes fragmented during drought, the deeper pools are often the last places where aquatic life can survive. Fish, insect larvae, crustaceans and other freshwater organisms become concentrated into these remaining pockets of water. In that sense, the pools are extraordinarily important: they are refuges in an otherwise drying landscape. But an isolated pool is no longer a stream! A flowing stream continually brings in new water, carries away waste products, replenishes oxygen and transports heat downstream. An isolated pool loses all of those advantages.

As the water becomes shallower, it can heat up rapidly in the sun. Evaporation reduces the volume of water still further, concentrating everything that remains in it. Plants, algae, bacteria and animals continue to respire, consuming oxygen. The pool can therefore enter a vicious circle: less water means warmer water and more oxygen stress. The greater biological stress the less oxygen available.

Warm water and disappearing oxygen

During the drought, temperatures in some of the remaining pools of Highland Water rose dramatically to 22°C, a temperature that will put particular stress on fish.  Temperature also controls water chemistry. Warm water holds less dissolved oxygen than cold water. At exactly the time when animals may be becoming more stressed by heat, and therefore potentially more vulnerable to oxygen shortage, there may actually be less oxygen available to them. At the same time, microorganisms continue to consume oxygen as they break down.

The extraordinary importance of reconnection

This is why the shingle banks in Highland Water are so interesting. The pools are gradually joining up again, and although that process may seem frustratingly slow, every connection matters. Once flowing water returns, oxygen can be replenished, cool water can move through the system, heat and metabolic waste can be carried away, and aquatic animals can move between previously isolated habitats. The pools cease to be isolated islands and become part of a river again.

That connectivity is hugely important after drought. The surviving animals in those pools are not necessarily starting again from nothing; they are potential sources for recolonisation as the stream recovers. So when a tiny channel begins to cut through a shingle bank, it should not be dismissed as insignificant. It may be the beginning of the reconnection of an entire stretch of river.

Drought doesn’t just remove water

This is perhaps the most important lesson from the demonstration on a stall. Drought is not simply about water disappearing. It changes the physical structure of the river, separating pools from one another; it changes the temperature of the remaining water; it changes the chemistry, including dissolved oxygen; and it changes the ecology, by preventing animals from moving through what was previously a continuous habitat.

The great shingle banks in Highland Water are therefore much more than piles of stones. They represent a temporary interruption in the river’s connectivity, and the slow process of breaching them is part of the river’s recovery.

So what happens next?

We don’t know exactly how long it will take Highland Water to fully reconnect. That will depend on rainfall, groundwater levels and, crucially, whether we get sufficiently large flows to move the gravel and reshape the channel. Some of the smaller barriers may disappear quite quickly once we have proper winter flows, while the largest deposits may take repeated high-flow events to rearrange.

This is something worth watching and measuring. We already have a record of what happened during the drought - including the extraordinary water temperatures - and now we can watch what happens as the river reconnects. Perhaps, over the coming months, we will be able to follow the story from

isolated pools to reconnection and to flowing water and recovery. Reconnection is already starting a short distance downstream.

Highland Water is showing us a much bigger story than a blue bottle experiment! When a river stops flowing, its water doesn’t simply become less plentiful. It becomes a different habitat. And when the river begins to flow again, the recovery is not simply the return of water. It is the return of a living, connected river.