Glossary

Alluvial (uh-lú-vee-uhl) refers to soil, sediment, or landforms that are formed over time and deposited by flowing water, typically by rivers, streams, or floodplains. These deposits are usually composed of a mix of sand, silt, clay, and gravel and are often fertile, well-drained, and deep, making them highly supportive of plant growth.

In the San Lorenzo Valley, alluvial soils are commonly found along creek bottoms, streamside terraces, and narrow floodplain benches—especially in low-gradient areas where sediment naturally accumulates. Examples include the lower parts of Ben Lomond, Felton, and Boulder Creek, as well as wider zones near confluences, such as where Bear Creek meets the San Lorenzo River.

These areas support high biodiversity due to their rich soils and fluctuating moisture availability and are home to species such as Western Sycamore (Platanus racemosa), Arroyo Willow (Salix lasiolepis), and Red Alder (Alnus rubra).

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