San Lorenzo Creek Watershed Archive

Key Watershed Terms

Bank — the sloping area between the water’s edge and level ground. Roots and vegetation stabilize the banks, filter sediments, and reduce soil erosion.

Buffer — a vegetated area of grass, shrubs, or trees designed to capture and filter runoff from surrounding land uses.

Coir — Coconut fiber used to cover restoration and erosion sites. It allows grasses and shrubs to grow through it while the fiber rots away.

Creek bed (or streambed) — the bottom of the creek (or any water channel), which is usually composed of a mixture of gravel, sand, and silt.

Creek channel (or stream channel) — the area of the riparian corridor that contains flowing water (either intermittently or continually).

Culvert — An underground water channel, usually placed under a road or structure to allow for development of land. Culverts take the form of concrete box-like structures or large-diameter storm drain pipes.

Deposition — settlement of materials from moving water onto the channel bed, banks, and floodplains. Deposition occurs when flowing water is unable to transport the material.

Estuary — Area where salt water (such as the bay) and fresh water (such as creeks) join, usually influenced by tides.

Floodplain — any lowland that borders a stream and is inundated periodically by its waters.

Groundwater — the water contained in the open spaces between individual soil particles. Below the ground surface and above the water table, water in the soil does not fill all the open spaces. Groundwater is not the same as surface water runoff.

Habitat — An area that provides food, water, shelter and space for wildlife.

Headwaters — uppermost reaches of a stream.

Intermittent stream (or seasonal stream) — a stream that flows only when there is sufficient stormwater runoff. Most creeks in East Bay watersheds are intermittent. Compare perennial stream.

Native — Something that evolved in this place over a long period of time, usually in reference to a plant or animal. California poppies and coast live oaks are native to this region. Eucalyptus trees, which were imported from Australia, are not native.

Non-native — something that evolved somewhere else. Eucalyptus trees evolved in Australia and were transplanted here. Non-native plants and animals are usually introduced by humans as they move about the land. Most of the grasses on the hillsides of the Bay Area are non-native. Vinca (periwinkle) started as a landscaping ornamental but now crowds out native plants on creek banks.

Outfall — the outlet of a body of water, especially in reference into a drain.

Perennial stream — a stream that flows continuously throughout the year. Compare intermittent stream.

pH — the measurement of the hydrogen ion concentrations in substances. Essentially this will tell you how basic (7.1 - 14 on the pH scale) or acidic(6.9 - 0) the water is. Healthy creeks usually range between 6.5 - 8.5. pH can be altered by the type of soil surrounding the creek, by car and industrial plant emissions (which contribute to acid rain), and by fertilizers or soil conditioners. Many species (especially trout) are very suseptible to changes in pH and can only live within narrow ranges.

Phosphates — a constituent of plants and animals (their wastes and bodies). Plants need phosphorus for growth and other organisms need it for metabolism. Phosphates are usually in low concentrations in nature since it either bonds quickly to soil or is used as soon as it is available. Excessive phosphates are usually due to human and animals wastes, industrial wastes, fertilizers, and soil erosion. Excessive levels can cause algal blooms that cloud the water and take away oxygen in the water which can lead to fish kills and other problems.

Pools — deeper portions of the creek where sediments have been scoured and water flows slowly. Pools are important habitat components for trout and other native fish.

Riffles — shallower areas in the creek where water flows quickly, often over gravel or rocks.

Riparian corridor — the vegetated area adjacent to (and including) the creek.

Riprap — a layer, facing, or protective mound of rubble or stones randomly placed to prevent erosion, scour, or sloughing of a structure or bank.

Run — the straight fast-moving section of a stream between riffles.

Runoff (surface runoff) — the portion of rain that moves over the ground toward a lower elevation and does not infiltrate the soil. "Urban runoff" refers largely to water from rain, irrigation, or industrial and household discharge that typically flows into a storm drain system that empties into a body of water (locally the San Francisco Bay). Runoff is not the same as groundwater.

Scour — concentrated erosive action of flowing water in streams that carries away material from the bed and banks.

Sediment — the soil particles in the creek. The sediment can be on the bottom of the creek or it can be suspended in the water. Water with a high sediment load (or turbidity) looks muddy or cloudy.

Sloughing — downward slipping of a mass of soil, moving as a unit usually with backward motion, down a bank. Sloughing is similar to a landslide.

Storm drain system — consists of street gutters, catch basins, underground pipes, open channels, culverts, and creeks. The storm drain system in the San Lorenzo Creek watershed is designed to carry this runoff directly into local creeks or the San Francisco Bay without treatment to remove pollutants. ("Only rain down the drain.")

Toe — the bottom of a slope or creek bank. (In the Alameda County Watercourse Protection Ordinance the toe is the starting point for calculation of setbacks from creeks; see section 13-12-320.)

Turbidity — the cloudiness (or clarity) of a water sample. If a great amount of suspended sediments (such as soil particles) are present in the water, the sample will be very turbid. Water clarity is important in that it helps keep the water temperature cool, indicates very little erosion in upstream areas, and is important for fish and insect reproduction cycles.

Upland zone — the area adjacent to a creek that extends away from the wetter riparian corridor.

Watershed — the land area that water flows across or through on its way to a creek, river, bay, or ocean. Rain that falls in the San Lorenzo Creek Watershed makes its way to one of nine major creeks. Bolinas, Castro Valley, Chabot, Cull, Crow, Eden Canyon, Hollis Canyon, Norris, and Palomares Creeks all empty into San Lorenzo Creek.

Woody debris — includes trunks and large branches of trees that have fallen into or alongside the creek.