Water Quantity

Importance to Salmonids

Each new generation of salmon is born within a freshwater stream system and the success of the generation is dependent upon appropriate stream flow. Flow rates affect all life stages, including the upstream migration of adults, survival of eggs, the emergence and viability of fry, and timing of smolt out-migration. To reach spawning grounds, adults require access to the stream system and sufficient water flow successfully to navigate passage impediments while migrating upstream. However, extremely high water velocities may prevent upstream migration until flow subsides. Coho salmon (Oncorhynchus kisutch) tend to spawn in small streams where the flow is 2.9–3.4 cfs. Coho and steelhead trout (O. mykiss) in San Mateo and Santa Cruz Counties migrate and spawn during the late fall and winter – a time of year when precipitation events are frequent and sometimes of high magnitude. Although these storms provide access to spawning grounds and the corresponding high water flow brings cool, highly oxygenated water to developing eggs and alevins, extreme storm events may also cause excavation of redds and wash young salmonids downstream.

While potentially deleterious to the earliest life stages, flood events are essential to the development and maintenance of healthy stream systems. Floods change the stream structure by altering the active channel, creating new side channels, and recruiting and transporting large woody debris. Flood events bring new sediment to replenish stream banks, and fresh seeds and propagules to colonize open soils. High flows carry coarse sediment and deposit gravel in downstream reaches while flushing fine sediment from spawning gravel. While a severe flood year may result in a low survival rate for the present year cohort of salmonids, the resulting habitat changes can create improved habitat for future generations. Because California's Mediterranean climate results in long rainless periods – usually between June and September of each year – a low flow period occurs in late summer through fall and can cause stream reaches to dry out into a series of pools. Low water levels concentrate juvenile salmon within pools, which increases competition for food and space while increasing predation risks. Low flow periods are often associated with higher water temperatures, an increase in migration barriers, reduced habitat, and decreased macroinvertebrate production – all factors that contribute to reduced salmonid production.

Human Impacts

Humans can impact the flow regime in three ways: controlling natural flow, increasing flow (adding water to the system), and decreasing flow (removing water from the system). While low summer flow may be a natural occurrence, human modifications, such as dams or water diversion and pumping, may drastically decrease the amount of available water. Human modifications to the flow regime also result in alteration in the timing and magnitude of flood events. Seasonal fluctuations are a natural process, with flood periods occurring during winter months and low or no flow periods in late summer and early fall. These natural extremes are important for maintaining habitat, but may create short-term detrimental conditions for spawning or rearing salmonids. Human modification of flow regimes can negatively affect salmonid populations. Alterations to the flow regime can result in a further reduction in flow during low flow events such as the dry season and a greater increase in flow during high flow events such as storms. Although water remains within the system, the timing, duration and intensity of flow may not provide suitable conditions for migration, spawning, or rearing.

Increased development in coastal counties, such as Santa Cruz, results in greater impervious surfaces (shown here in white) which alter proper drainage to groundwater and can disrupt normal stream flow. Streams in close proximity to impervious surfaces, such as paved roads, are especially at risk.

Human disturbance within the watershed can result in an increase in water flow by enhancing the amount of water in a stream or stream reach. As precipitation falls to the Earth's surface, it becomes available for use by vegetation, enters the ground to become part of the ground water system, or is evaporated back to the atmosphere (see the Hydrology section for more information). Paving or compacting soils – creating impervious surfaces – prevents water from entering the soil, thus cutting off water supplies to vegetation and groundwater. Since the water cannot enter the soil, it is not available to plants and it cannot replenish groundwater supplies. While impervious surfaces have a high evaporation rate, this rate is quickly surpassed during moderate and high precipitation events. The large quantities of water that run off impervious surfaces are often controlled through an engineered system of culverts and pipes which result in a more centralized and higher input of water than would occur if precipitation were allowed to seep into soils naturally. Increased flow may also result from removing vegetation. Plants use water for photosynthesis and release water back to the atmosphere through transpiration (see the Hydrology section). They also prevent soil compaction by preventing the direct impact of heavy storm waters on open soil and by loosening soil through root action. Non-compacted soil can absorb soil water more efficiently, resulting in reduced runoff. Although salmon require high flows to reach spawning grounds, very high flows prevent migration. By increasing flow, especially during storm events, human modification to the watershed may prevent successful migration, excavate redds (spawning sites), and wash juveniles downstream.

As human population increases within a watershed, demands for water intensify. Water is needed for residential, commercial, and agricultural purposes and is usually supplied to land owners through municipality or private water diversions and wells. While water diversions directly remove water and reduce flow, wells may also impact the amount of available water within the stream. Groundwater contributes to interaction with stream flow and is responsible for the majority of summer flows in San Mateo and Santa Cruz Counties. Wells can deplete available groundwater stores and reduce the amount of water available for stream flow. Reduced groundwater resources may also remove water from stream systems (see the Hydrology section). During winter months, reduced stream flow can be highly detrimental by preventing access to spawning grounds through barriers or poor conditions, dewatering established redds, and stranding fry. Low summer flows may result in dried-out stream reaches, stranded juveniles, and reduced habitat quality.

A significant impact caused by human change to stream flow is its effect upon sedimentation. Both increases and decreases in flow can cause increased sedimentation. For example, increases in magnitude and frequency of flooding can cause a channel to deepen and widen, causing bank erosion. Decreases in flood events and low flows result in deposition as fine sediments settle out of the slower water. This sedimentation results in the filling in of pools and spawning gravel. As discussed in the Substrate / Sediment section above, sedimentation, especially increases in fine sediments (silts), can have negative impacts upon salmonid production and survival.

Local References

Pajaro Valley Water Management Agency. 2003. "Revised Basin Management Plan."

State Water Resources Control Board (SWRCB). 1993. San Gregorio Creek Adjudication Decree No. 355792, San Gregorio Creek Stream System in San Mateo County, California.

General Reference

California Department of Fish and Game (CDFG). 2002. "Status Review of California Coho Salmon North of San Francisco." Report to the California Fish and Game Commission. 336 pp. View on-line document.

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