A river that shaped the village
The Woluwe rises in Boitsfort, in the Sonian Forest, and flows into the Senne at Vilvoorde. It gave its name to Woluwe-Saint-Lambert, Woluwe-Saint-Pierre and Sint-Stevens-Woluwe, formed the western border of Zaventem and runs right through Diegem. For centuries the village lay on both banks. Even today most people in Diegem live on the two slopes along the Woluwelaan, between the Saventerlo and the Korenberg on one side and the Drielinden plateau on the other.
In the mid-19th century Alphonse Wauters described the valley as a jewel: a footpath followed every bend of the river, past whitewashed cottages, kitchen gardens, meadows and ponds lined with alders. As late as the 1950s townspeople came here for a Sunday walk, to the St.-Katarinamolen, the Donjon, the castle and the church.
The Woluwe and the Hollebeek
Alongside the Woluwe ran a smaller stream, the Hollebeek: a 'hollow' or open ditch. In Diegem the two watercourses met and parted three times. The millers made clever use of this: with sluices they sent the water into whichever stream had to drive their mill wheel. Old deeds also call the Woluwe 'Leybeke' or 'Moolenbeeck', and a tax had to be paid for the many footbridges, gutters and culverts over the water.
In 1208 the Woluwe was diverted at Vilvoorde through a dug canal into the centre of that town, to supply the ducal mills with water. As a result the abbey of Grimbergen lost its water mill in Diegem. Duke Henry I compensated it with tithes in Wemmel.
Floods and pollution
The river was not only a blessing. In the night of 4 to 5 May 1839 it burst its banks and damaged several houses; according to the town clerk the water rose 9 to 10 feet and a block of six houses was destroyed. From 1846 the mayor complained about waste from factories upstream, including gas residue from Zaventem: the water stank and the cattle fell ill. In 1884 Georges De Coninck and Zunz had to pay a tax because they polluted the Woluwe.
Underground
In 1924 Diegem joined the intermunicipal company for the sanitation of the Woluwe valley. In 1930-1935 the river was covered over: the trees along the banks were felled and pile drivers thundered through the valley. After the war the roof of the new sewer was asphalted, and so the Woluwelaan came into being. Only at the Duivenmolen and just across the border, at the Stockmans mill in Zaventem, can the Woluwe still be seen.
Between 1973 and 1975 the Woluwe intermunicipal company built a 48,000 m³ storm basin at the corner of the Tenaertslaan and the Woluwelaan. In heavy rain it holds back part of the water, so that Machelen and Vilvoorde flood less often. It is one of ten such basins between Auderghem and Machelen; the one in Diegem cost about 100 million francs.