text: On Thursdays, the pool is closed for cleaning from 9 a.m. until 7 p.m.  The floodgates are sometimes partially opened, lowering the water level in the pool by several feet.  Employees then buff or blast pressured water against the limestone bottom of the pool's shallow end, as well as the steps and ramps leading into the water, in order to wash away the hazardous and slippery algae buildup. In the deep end, a fire hose is used to push debris toward the downstream dam. Overgrown vegetation is also trimmed on cleaning days. The pool is closed for several weeks once a year for more extensive cleaning.
  The pool is closed during flash flood warnings as Barton Creek may flood and overflow the diversion dam.  Swimming in Barton Springs Pool is then unsafe, as it transforms into a raging creek. The pool is then closed for several days after a flood, so mud and debris that washed into the pool can be removed. Other unplanned reasons for pool closure include lightning or thunder in the area, "search and rescue" situations, serious medical emergencies, high fecal coliform count, and chemical spills (either inside the pool itself or over the Barton Springs segment of the Edwards Aquifer).
  Environmental issues
  The pool has been closed to the public a number of times since the 1980s due to unsafe levels of fecal coliform (E. coli) bacteria in its waters.  The source of contamination is still undetermined: many point to upstream urban development as the cause, although others note that high bacteria levels were seen in the 1950s, when development was less pervasive. Contamination is usually worse after heavy rains due to the flushing of the effluent of upstream subdivisions into the Edwards Aquifer recharge zone which feeds the springs.