These small rivers tend to have a considerable fall over their length, usually about .75m per kilometre. This means that they are ideal for powering water driven mills, and as a consequence the rivers are controlled by weirs (barrages in French) to create header ponds. The flow is then diverted down a mill stream (bief in French), creating an island. The mill may be situated on the island or on the bank. Very few of these mills now function, but property owners are responsible for maintaining and operating the sluices as necessary to control flow and levels.
Mills have been sited at roughly 1km intervals on these rivers since about the 12th century, and the ecology of these small rivers and mill streams is now dependent on the controls associated with the mill workings. The many weirs make the rivers seem bigger than they actually are, and slow the flow down. Although these rivers are weired, they are rarely canalised, and the banks are well vegetated. In recent decades, the banks of these small rivers have become more and more overgrown, restricting access to the rivers, and causing difficulties in the dispersal of riparian species into the wider landscape (eg emerging dragonflies).
As part of a new river management scheme, overseen by local river technicians, the riverbank vegetation is being systematically thinned and the over abundant poplars removed. In some places weirs are being removed in order to speed up flow and ensure the water is better oxygenated. This is proving rather controversial, as the principle beneficiaries are the angling clubs, who sometimes do not see the wider ecological picture.
Water quality is generally good, although in many places water meadows have been ploughed up. This practice, and those such as intensive dairy farming mean that the water is periodically polluted by silt and agricultural runoff of one sort or another.
There are some problems with the invasive alien species Water Primrose Ludwigia spp (jussie in French). This aquatic plant rapidly covers the surface of still or slow moving water, out competing native plants, blocking out light, causing an accumulation of organic material and interfering with the breeding of fish and aquatic insects. The river technicians are dealing with this by manually weeding.
There are also problems with the invasive alien species Coypu Myocaster coypus, whose burrows undermine river banks. The animals have been declared vermin (nuisible in French) and landholders have an obligation to notify the authorities and eradicate animals.
For an overview of the management programme for the Claise River, see Une Belle Riviere, a blog post (in English) on Days on the Claise.
|The Claise, typical of the small Tourangelle rivers, here in December swelled by winter rain and just bursting its banks. The island to the left is formed by the remains of a medieval bridge, the weir in the distance creates a mill pond for the Moulin de Chanvre ('Hemp Mill') and the millstream is the channel going off to the right. The photograph was taken from the boat ramp and this is a good spot for watching Banded Demoiselle Calopteryx splendens and Black-tailed Skimmer Orthetrum cancellatum in the summer.|