The riu rau was a rectangular building with arches to allow quick access to a sheltered area for grapes which were laid outside in the sun on canes to dry. Drying the grapes of course was part of the process of making raisins. At night to protect the drying grapes from dew the grapes were lifted under the riu rau. And the open arches made it easy to do this rapidly if it rained during the day.
The photo shows a modern version of the riu rau. It is part of a small market garden surrounded by vines near the village where I live. The rows of grotesque shapes on the higher and lower levels (terraces) in front of the riu rau are the vines at the time of writing in mid-April. This little vineyard is probably kept for the owner's enjoyment of the tradition rather than any profit. The roof of his house behind the riu rau can be seen in the photo as well as the garage with the blue door to the right.
The riu rau structure originated in the late 18th or early 19th century. There are not many to be seen now as this form of agriculture is no longer profitable here on the Costa Blanca. Apparently there are no taxes on using an old riu rau as part of a modernised building: an incentive to keep the attractive shape as a feature of the landscape. Fortunately this pleasing form has also been used by some local builders for modern houses like Casa Kaduna (below) and other houses nearby.
The photo above shows the riu rau effect adapted to a modern house.
Although raisins are no longer a local product the grapes certainly are. The photo below shows a vine in Spring with the new growth sprouting from the knarled wood. If you have seen a French vineyard you will note a remarkable difference here in Spain in the way the grapes are grown. They are much closer to the ground; the parent vines are kept low and the new growth is not given a supporting structure for the new growth to spread along.
Flora, my wife, once told the grandchildren that there were reindeer farms all around us. You can see what she means when in autumn and winter after the grapes and the new vine growth have been removed then the rows of Spanish vines look like antlers pushing through the ground!
Each spring the countryside around our village looks as in this last photo with the dark wood of the lines of vines, the dark green of the olive trees, and the pink or white blossom of the almonds.
Bibliography: Vicente Torres at http://blogs.periodistadigital.com/puntodevista.php/2007/10/30/el-riu-rau-bien-a-proteger
Vicente García Morant and Juan Bta. Padros Martínez 'Historia del Poble Nou de Benitachell', 1976