Around the year 2000, populations of inconspicuous, 5-mm hydrobiid snails were discovered at several semi-isolated marshes around San Francisco Bay, California, where freshwater meets salt water. Coring indicated these are native species, including Tryonia porrecta (Mighels, 1845). The present, long-term monitoring at some of these sites with and without these small snails suggested that these abundances tend to be quite stable, through present 2014 sampling. But the recent, record drought since 2012 risked high salinities in these vulnerable brackish areas, including a site with the normally rare California Brackish Water Snail, Tryonia imitator (Pilsbry, 1899). Sites in presently 100% seawater at Elkhorn Slough on nearby Monterey Bay, once with frequent T. imitator brackish water snails yielded only dead shells during this past 15 years, among large, introduced predatory snails. Yet a more isolated site nearby at 50% seawater yielded small (3 mm) T. imitator brackish water snails observed (live) on green algae among pickleweed, at warm (30 °C) water temperatures, with no other gastropod species, soon after rare rains during the drought, in April, 2014. Isolated marshes may have advantages in slowing colonization of invasive species. Despite vulnerability of semi-isolated brackish-water sites to elevated salt during drought, some sites maintained these native snails (without invasive snails) even after drought.