Where Stevens Creek approaches the marshes at the edge of San Francisco Bay, flowing past several high-tech business parks, mobile home parks, and the NASA Ames Research Center, it widens out and starts becoming a tidal slough. Along the trail one can see snails if one knows where to look and what to look for.
These snails are mainly the milk snail, Otala lactea, and also the white garden snail, Theba pisana. Both are non-native species originally from the Mediterranean and have become naturalized along local creeks whose headwaters arise in limestone-rich mountains to the south, providing a source of all-important calcium for maintaining their shells. I have not seen these snails anywhere except along the banks of such creeks.
Here's some pics I took last year during a walk along the Stevens Creek Trail.
Looking across the creek bed, choked with vegetation.
Here are a couple of milk snails on the stems of another invasive non-native, fennel.
This pic shows how much variation can occur in the colors and patterns on milk snail shells. No two are exactly alike, and each one is beautiful.
Close to Shoreline at Mountain View, clusters of snails can be found estivating under the lids of storage containers behind a parking lot. This bunch appears to be a mix of milk snails and white garden snails.
Many different ages of snails are nestled into this crevice of a storage container.
The snails slow down their metabolisms, seal up their shells with dried mucus to conserve moisture and wait out the hot dry summers until the cool winter rains begin. Not all will survive these long months of dormancy.