The floodplain of the Avon has an altitude of about above sea level, although the city centre is at an elevation of around above sea level. The river, once an unnavigable series of braided streams broken up by swamps and ponds, has been managed by weirs into a single channel. Periodic flooding, which shortened the life of many buildings in the lowest part of the city, was normal until major flood control works were completed in the 1970s. Kensington Meadows is an area of mixed woodland and open meadow next to the river which has been designated as a local nature reserve.