A coalition of environmental groups Monday announced plans to sue a regional water treatment authority and the cities of San Bernardino and Colton over the repeated stranding and killing of Santa Ana suckers, a fish on the federal threatened species list.

Roughly once a month, a water treatment plant that is jointly owned by the cities halts its outflows, quickly reducing a drought-stricken stretch of the Santa Ana River to a ribbon of dry gravel and stranding thousands of suckers.