Look: 10 Dangerous Colorado Rivers and Lakes You Should Know The following lakes and rivers have earned a reputation for catching visitors off guard. Scroll through the list and find out why knowing the risks could make your next adventure much safer. Water emergencies in Colorado can occur when people pay more attention to the weather than to water conditions. 90-degree days don’t make rivers safe to swim, and calm reservoirs can become dangerous during afternoon winds. PLAY OUTDOORS: See All 43 of Colorado's Beautiful State Parks There are 43 State Parks open to the public in Colorado.