Passage: For more than 1000 years, the inhabitants of Bahnasa dumped garbage at a series of sites out in the desert sands beyond the town limits. The fact that the town was built on a canal rather than on the Nile itself was important, because this meant that the area did not flood every year with the rising of the river, as did the districts along the riverbank. When the canals dried up, the water table fell and never rose again. The area west of the Nile has virtually no rain, so the garbage dumps of Oxyrhynchus were gradually covered with sand and were forgotten for another 1000 years.

Question: What  happened to the districts along the riverbank every year?
did not flood every year