Record drought from 2012 to 2016 followed by rainfall in the winter of 2017 provided an opportunity to examine how changing climate conditions may affect migration opportunities for the endangered southern steelhead trout (Oncorhynchus mykiss). This study examined how intermittently open estuary-ocean interfaces in the Santa Monica Bay that have historically supported steelhead evolved temporally and volumetrically. All seven lagoons in the study area breached by January 2017 after five years of drought and nearly exclusively closed conditions. Duration of breach was affected by the size of the lagoon, with smaller lagoons remaining breached longer than larger lagoons. Conversely, volume capacity persisted longer in larger lagoons. Lagoon condition was quantified by presence/absence of breach and passibility, coupled with daily rainfall. This study provides important lagoon planning, restoration and management information needed to support recovery of southern steelhead trout populations in the face of climate change.