Tip stability is affected by water conditions. Tips 4, 5 and 7 had been sited on streams or springs. The presence of the springs was common knowledge in the area, and they had been marked on the Ordnance Survey and Geological Society maps since 1874. Tip 4 at Aberfan, which had been used between 1933 and 1945, was large, and had been started on boggy ground between two streams. At the time of its planning, the Merthyr Tydfil borough engineer thought that despite the position, it would be unlikely to avalanche. Following some ground movements in the tip in the early 1940s, a drainage channel was dug in early 1944. In November that year part of the tip slid down the mountain to stop approximately above the village. In May 1963 Tip 7 shifted slightly; in November that year there was a more substantial slide. The NCB stated that the movement had not been a "slide", but was instead a "tailings run"—a run-off of tailings from the surface of the tip—which left its stability unaffected. After the slide, the NCB stopped tipping tailings on number 7, but normal spoil continued to be deposited.