3.3 - Soils

The USDA Soil Conservation Service Soil Survey of Western Alameda County was used to delineate the soil types of the upper Strawberry Creek watershed. The soil types are presented in Table 2 and shown on Figure 3. Soil series may be assessed for their runoff potential, risk of erosion, and other parameters of concern in the management of Strawberry Creek and its watershed. In general, the upper watershed soils are highly impermeable and have a high runoff potential as well as a high risk of erosion. Strawberry Canyon is the site of numerous landslide bodies that will easily slide when undercut or flow when saturated. Development on the Canyon soils is severely constrained by steep slopes and shallow depth to bedrock.

Hydrologic soil groups as defined by the SCS can be used to estimate runoff potential of each soil type in the watershed based upon its infiltration capacity. Infiltration rates decrease and surface runoff potential increases as soil types are classified A through D. Approximately 38 % (431 ac) of the watershed consists of soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. Soils having a very slow to slow infiltration rate comprise about 23% (234 ac) of the watershed, whereas 1 % (18 ac) of all the watershed soils have a slow infiltration rate. Soils with a very slow to moderate infiltration rate account for 20% (234 ac) of the watershed. The remaining 18 % (209 ac) of the watershed area is unsuited to the hydrologic soil classification system because it is covered by urban structures or comprised of heterogeneous artificial fill materials.

Three different soil types account for 75% (736 ac) of the upper Strawberry Creek watershed area. Maymen loam is the predominant soil type in the watershed (366 ac or 32%). This is a shallow (10-20 in) somewhat excessively drained soil with rapid to very rapid runoff potential and high to very high risk of erosion. It is found on upland areas with slopes of 30-75%. The pH range of Maymen loam is strongly acid (4.5-6.5). This soil type formed in material that weathered from sedimentary rock. It is generally underlain by sandstone, siltstone, and conglomerate.

The Maymen-Los Gatos complex is another dominant soil type in the watershed, comprising 265 acres (23%). This complex consists of steep and very steep soils on uplands. Slopes range from 30-75%, but are mainly 50-75%. This soil complex is composed of 50% Maymen soils and 35% Los Gatos soils. The remaining 15% of this soil type are small areas of Millsholm silt loam and some rock outcrop. Depth to bedrock ranges from shallow to moderately deep (10-40 in). Runoff and erosion characteristics are identical to Maymen loam. The pH of this complex ranges from strongly acid to neutral (4.5-7.3). The Maymen-Los GatoS'complex formed in material that weathered from sedimentary rock and is underlain predominantly by sandstone and shale.

The last major soil type is the Xerorthents-Millsholm complex which comprises 20% of the watershed area. This complex is on hills at slopes ranging from 30-75%. It consists of about 70% loamy Xerorthents (altered soil or fill material), 20% Millsholm clay, and 10% of small areas of Maymen loam, Los Gatos loam, and Los Gatos silty clay loam. Depth to bedrock ranges from shallow (10-20 in) to over 20 inches. This complex is well to somewhat excessively drained. Runoff is rapid to very rapid and risk of erosion is high to very high. The pH of this complex ranges from medium acid to slightly alkaline (5.6-7.8). The xerorthents in this complex consist of soil materials that have been altered by cutting or filling for urban development and, as a result, they have variable soil characteristics. The Millsholm soil formed in material that weathered from fine-grained sandstone. This soil complex is generally underlain by sandstone, siltstone, and undivided Quaternary deposits.