Fishes of California coastal streams and associated coastal lagoons have adapted to the Mediterranean-style rainfall cycle. Winter rains open the lagoons to the ocean; subsequent lack of rain and seasonal changes in beach dynamics typically closes them for much of the year. Dry and wet season artificial breaching or opening of barrier sand berms has been suspected to disrupt fish populations and lead to mortality of many aquatic organisms including federally endangered species. Such breaches have been rarely observed and then only after at least a few days or more have passed. Artificial breachings of three lagoons have been observed during or within a few hours after breaching and provide documentation of extensive disruption and mortality of aquatic organisms. These observations, Aliso Creek, Orange County (1975), Santa Clara River, Ventura County (2010), and Corcoran Lagoon, Santa Cruz County (2014-2015), confirmed many changes and effects of these events, including mortality of the federally endangered northern and southern tidewater gobies and southern steelhead. Despite the many ostensibly beneficial and non-faunal related reasons for breaching, our observations confirm such actions can cause considerable mortality of threatened and endangered species and are probably more severe than natural wet season breachings. Many city, county, as well as state and federal laws provide regulation of lagoon breaching to protect habitat and minimize or mitigate for impact to sensitive species and these need to be maintained and strengthened.