Jewels in the crust
Any large and permanent body of fresh water is a very precious thing, with all sorts of uses and reasons to be valued, truly a jewel in the crust of the Earth. A total of about 90,000 cubic km of the world’s fresh water lies within some five million lakes at any one time. This adds up to a formidable part of the biosphere and a huge resource for people to use, and not all of these uses cause problems for people or for nature.
Lakes form where the water draining off an area of land is blocked by something, whether a shelf of hard rock that makes groundwater overflow onto the surface, or hilly terrain that denies a river its path to the sea, until it gets deep enough to find a new one. All lakes have drainage basins or catchments – the area where rain is destined to flow into the lake.
In this area, anything washed or leached from the land in the basin finds its way into the lake, whether this is silt from an eroding landscape, salts from underground rocks, or agricultural chemicals from a farming zone. Lakes delay that water, giving whatever it contains time to fall as sediment to its bed, or to affect the living ecosystem of the lake itself.
So a lake is like a sump, where all things stay for a while and where many of them end up.
Any lake, therefore, is subject to several processes, which may be in or out of balance, beneficial or destructive. The amount of water entering the lake relative to the amount escaping decides how big or deep it will be, how long its water must remain there, how much will evaporate, and how salty it will become as a result. These balances will change over time, seasonally or in the longer term, as rainfall alters in the catchment, making the lake shrink or expand, become saltier or fresher.
Likewise, if rivers flowing into the lake are diverted for irrigation, then more will evaporate from farmland and less will reach the lake. If the irrigated fields are sprayed or fertilised, then these chemicals, or their derivatives, will also reach the lake, along with the reduced, concentrated water flow. If, on the other hand, rivers are diverted entirely out of the drainage basin, then the lake will dry up, fast.
And then, if there’s a city on the lake shore, or on a river flowing into it, other factors kick in. Sewage, for example, and the oily effluent of car washes, the soapy traces of household detergents, and the accumulated trash of street litter or escaped material from garbage dumps.
Or, if there are boats and ferries plying the lake, there’ll be diesel leaks and probably fishing too, with all its disturbance to the local web of life. Or maybe a minister has decreed that new species be introduced to the lake, to ‘improve’ fishing. Then, maybe, there’ll be cataclysmic ecological shifts as foreign catfish fight it out with native cichlids, or exotic crayfish with indigenous clams.
Still, it’s important to see lakes as dynamic concentrations of benefits, options, and opportunities, as well as sources of contest and conflict between the various uses that the divided minds of humans may dream up. Many lakes, for example, are home to immense populations of water birds, either permanently or during seasonal migrations. These can attract wildfowl enthusiasts, birdwatchers or bird hunters, and either can bring great financial benefits to the people who live around the lake, as well as encouraging more ecological and sympathetic management of the lake ecosystem itself.
Read on: River water.