But how do the creatures that live in the water cope with its inexorable flow downstream, especially in the upper reaches of streams where that flow can be as fast and furious as any lowland flood?
Take the plethora of invertebrates that inhabit streams - from the larvae of stoneflies and caddis flies to mites, beetles and flatworms. Most live on the river or streambed, among gravel and stones or in mud.
The paucity of actively swimming or floating aquatic invertebrates is believed to be an example of evolution in action. In other words, if you live in a medium that constantly moves past you, it pays to keep your head down (and the rest of you, for that matter). Otherwise you get carried away or use up considerable energy battling to stay where you are.
All the same, large numbers of water invertebrates get carried away, so to speak, daily. Just how many we now know, thanks to research by Dudley and Nancy Williams of the University of Toronto. They obtained Canadian government funding to study such processes in a tiny mountain stream in Snowdonia.
Using specially designed traps, they sampled counts of the numbers of streambed invertebrates being carried downstream as well as those struggling back upstream against the current. Subtracting one count from the other gave the net loss of such invertebrates downstream.
Near the stream's source, the Williamses put a net loss at almost 262,000 invertebrates each year. Only 19 per cent of those lost downstream were replaced by upstream movement of other invertebrates.
At a sample site 200 metres downstream, the net loss per year was more than 40,000. Here, more than 55 per cent of those lost in the flow were replaced by others making a heroic comeback. The difference over such a short distance is explicable, in part, by such factors as water depth and the gradient of the stream section.
What is not so clearly explained is how such losses have not long ago resulted in at least the headwaters of streams being drained completely of their invertebrate life.
The theory, borne out only partially by bits and pieces of evidence from other studies, is that the washed- away invertebrates, many in larval form, mature into their flighted adult forms and fly, double-quick, back to the headwaters to lay their eggs.
Sounds neat. The Williamses used flight traps to try to measure the direction such flights took. The results were mixed. For instance, in none of the eight common species of stonefly did females show a strong overall upstream flight preference. Indeed, females of two species took a particular shine to flying downstream.
Caddis flies behaved no more akin to the theory. Most showed no clear flight preference upstream and no correlation between their flight direction and local wind direction. There was evidence, at least for one species, that some female caddis flies flying upstream contained more - and larger - eggs, suggesting that those making a success of their trip may be more effective at producing the next generation.
Overall, the adults of six out of nine important aquatic insect groups living in the stream showed strong upstream-flight tendencies at one or more of three trapping sites. But the trend was often haphazard between sites. There were also examples of flying well beyond the call of duty - or necessity anyway. For example, in four families of fly, adults flew beyond the source of the stream, denying any replacement possibilities, unless, that is, they turned back when unobserved.
But this seeming inability to return whence they came was not common to every invertebrate living in the little stream. Take the water shrimp Gammarus pulex, the most abundant non-insect invertebrate in the stream. Losses downstream were substantial. Yet, at several times during the year, the numbers crawling back upstream approached, and sometimes exceeded, those drifting seawards. Some other stream invertebrates - blackflies, riffle beetles and flatworms, for example - also made sterling efforts.
Nevertheless, the counts do not explain reliably how all stream invertebrates manage to maintain their populations in the headwaters. It may be that only a small proportion of egg-carrying females need to return in order to produce enough larvae to replenish numbers.
And the random nature of many adults' flight paths, if replicated around the headwaters of all the streams in an area of upland, could mean that there is some exchange of invertebrates between streams sharing similar ecological conditions. So what seems somewhat haphazard flight paths may in the long term be a way of spreading survival risks; after all, if they were not successful, our streams would be completely devoid of life.
Source: Freshwater Biology, 30:199.