The Kelp Forest Monitoring data record span surveys across 24 years from 1999 through 2023 at 20 locations on the Sonoma-Mendocino Coast, Northern California. Years without data, inclusive: 2002, 2020, 2021. These surveys are ongoing and are conducted by the California Department of Fish and Wildlife dive team with participation from dive program partners at UC Davis, UC Santa Cruz, Cal Poly Humboldt, Sonoma State and other dive programs and volunteers. Not all sites were surveyed in all years. Surveys prior to 2003 were not conducted by the same teams or with the same methods except that all surveys were done using Scuba along 30 x 2m transects randomly placed in the subtidal zone in rocky habitats dominated by bull kelp, Nereocystis luetkeana, forests. These randomly placed band transects surveys were stratified by depth (A=0-15, B=16-30, C=31-45, D=46-60 ft) as we know sea urchin and abalone populations differ by depth. Data collected for algal habitat coverage from 17 years of those surveys (1999-2016) include quantification by percent coverage of six substrate/algal types: bare rock, canopy, encrusting, foliose, subcanopy, and turf. These data provide a baseline of algal coverage in the kelp forest before, during and after the major marine heatwave of 2014-2016 in northern California, Sonoma and Mendocino counties. Algal composition in the subtidal has changed dramatically from the time before 2014 to post marine heatwave and sea urchin population explosion. The substrate is now dominated by encrusting algae including crustose coralline algae, Ralfsia, Hildenbrandia, and other fleshy crusts.