The Colorado through Grand Canyon now lacks the source of sediment it needs to build sandbars and islands, and these natural fluvial formations within the canyon have now suffered severe damage from erosion. The floods that once scoured the river each year are now contained behind the dam except in extraordinary cases such as 1983–84; the lack of floods has promoted vegetation encroachment which not only has considerably changed the riparian zone environment but has created problems for tourism, as hikers and boaters often cannot find good spots to camp due to overgrowth. Flood control has also caused an inability of the river to carry away the rockslides that are common along the canyons, leading to the creation of incrementally dangerous rapids that pose a hazard to fish and boaters alike. Before damming, the Colorado commonly reached flows of more than during the spring; this has been limited to less than most years with few exceptions.