in turn feeds the hydroelectric power stations. Some of the water diverted by the dams is used for irrigation. Mirabeau to Avignon The valley narrows for a few kilometres until the water gap at Mirabeau, at a depth of , then widens again into an even broader plain until the confluence with the Rhône south of Avignon. Its direction changes from southerly to westerly then northwesterly, aligning with the small Provençal mountain ranges between which it flows (Alpilles and Luberon). The Durance receives only one significant tributary on this last part of its course: the Calavon, which flows around the Lubéron range to the north. Summary of tributaries This is a list of rivers longer than that flow into the Durance. They are listed in order of the confluence, starting upstream. (L) left bank tributary; (R) right bank tributary; (MR) main river, the name given to part of the water course taken into account in the calculation of its total length. Hydrology A river is known as "capricious", alternating between its feared flash floods (it was called the third plague of Provence) and its low water levels. The upper Durance is an alpine river with a flow ranging from . Its total drainage area is . At the confluence with Ubaye, salmon used to thrive, and trout were found up as far as Sisteron, before the development of the river. Later it becomes a Mediterranean river with the characteristic morphology. Flow The sources of the water are a combination of melting snow and the drainage of rain from the surrounding hills and plateaux which have a Mediterranean climate. The latter's tributaries bring rain water mainly in spring, autumn and winter but little during the summer. At Serre-Ponçon, its basin of can create a flow as low as and a maximum flood of (value recorded in 1957). At the confluence with the Rhône, the average natural flow of the Durance is approximately , with a high annual variability. It can vary between (the most severe low water levels) and in catastrophic historic floods, levels reached in 1843, 1882 and 1886. At the outlet into the Lac de Serre-Ponçon, the medium flow is ; at Oraison it is and after the confluence with the Verdon, flow reaches (250 to 330 m³/s in spring, 100 m³/s in the summer). The contribution of water from the downstream tributaries is very low. The annual maximum