Reading Passage 2: Rivers and Small Dams
Large dams attract most of the public attention, yet they are vastly outnumbered by small ones. Across Europe alone, surveys suggest that there are well over a million barriers on rivers and streams, and the great majority are less than five metres high. Many were built centuries ago to power mills, to divert water into fields or to create ponds for fish and ice. Others are more recent: low concrete steps, known as weirs, were installed to control flooding or to measure the flow of water. A large proportion of these structures no longer serve any purpose at all, but they continue to shape the rivers on which they stand.
Their combined effect can be considerable. A barrier only a metre high may be impossible for many fish to climb, and species that need to travel upstream to breed, such as salmon, trout and eels, are cut off from the gravel beds where they lay their eggs. Above each dam, the water slows down and becomes warmer, and the fine sediment that would normally be carried downstream settles on the riverbed, smothering the spaces between stones in which insect larvae live. Below the dam, by contrast, the river may be starved of gravel and sand, so that its bed and banks are gradually worn away. According to the freshwater ecologist Marta Lindqvist, these changes are easy to overlook precisely because each structure is so modest: 'No single weir looks like an environmental problem. It is the thousands of them together that turn a river into a chain of ponds.'
Over the past three decades, removing obsolete barriers has become an increasingly common way of restoring rivers. The results have often been striking. On one river in the north of Spain, where a series of six old mill weirs was taken out over four years, migratory fish were recorded upstream of the former dam sites within two breeding seasons, and the number of insect species on the riverbed roughly doubled over the following decade. Similar recoveries have been reported elsewhere, though the speed varies greatly depending on how far fish have to travel and whether a breeding population still exists nearby.
Removal is not without risks, however. Sediment that has built up behind a dam for a century or more is suddenly released, and in the first months after removal the water downstream may become cloudy, burying spawning grounds and blocking water intakes. Where the old mill ponds lie downstream of industrial sites, the trapped mud may also contain heavy metals or other pollutants. The hydrologist Daniel Okoro, who has monitored more than forty removal projects, argues that these problems are usually short-lived. 'In most cases the river redistributes the sediment within one or two years of high flows,' he says, 'but the exceptions are serious enough that every site needs to be tested before work begins.'
A further complication is that barriers sometimes have unexpected benefits. In parts of North America, low dams prevent invasive fish species from moving upstream into waters where native species survive, and conservation managers there have occasionally chosen to keep, or even build, barriers for this reason. Some dams also protect populations of rare freshwater mussels that have adapted to the slow, deep water of a mill pond. For the conservation biologist Helen Marchetti, this shows that removal should not be treated as an automatic good. 'A barrier that is harmful on one river may be a refuge on another,' she points out. 'The decision has to rest on what is living there now, not on a general principle.'
Local communities can also be resistant. Mill ponds have often become popular places for walking, fishing and swimming, and residents may be attached to the view of water behind an old stone weir. Some structures are protected as part of a region's industrial heritage. The historian Arno Velde, who has studied disputes over several removal schemes, has found that opposition is strongest where people are informed late in the process. 'When residents learn about a scheme from a notice on a lamp post, they assume the decision has already been made,' he notes. 'Where they are involved from the start, many become its strongest supporters.'
For these reasons, full removal is not the only option. Fish passes, a series of small pools or a sloping channel that allows fish to bypass the barrier, can be built beside a weir that needs to remain. Partial removal, in which a gap is cut in the middle of a structure, can restore much of the natural flow while keeping part of the historic stonework visible. Neither approach is as effective as removing a barrier completely, and fish passes in particular require regular maintenance. Even so, with so many barriers to deal with and limited funds available, restoring rivers is likely to depend on a mixture of methods chosen to suit each site.
