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Glossary
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- ADCP
- Acoustic Doppler current profiler. Sends acoustic signals into the water column, bouncing the signal off of particles, to estimate water velocity.
- advection
- The transport of any substance by the motion of a fluid that contains the substance.
- advective flux
- The portion of flux contributed by the temporal mean motion of water and average concentration of a substance.
- bathymetry
- The spatial variation of depth of a body of water.
- benthos
- General term for the sediment bed below the sediment-water interface. Usually populated with benthic organisms within the sediment bed (mollusks, worms, etc.).
- clay
- Hydrated aluminum silicates, which take flat, plate-like shapes. Electrostatic forces causes individual clay particles to stick together. Individual particles may be as small as 0.001 mm.
- cohesive sediment
- Clay or silt particles that adhere to each other due to electrostatic forces or "stickiness" caused by EPS.
- Delta
- Generally, a depositional area located near the confluence of rivers and oceans. In California, the Delta refers to the complex mesh of rivers, canals, export pumps, and control gates located between the Sacramento and San Joaquin Rivers. Water from the Delta is exported to Southern California for agricultural and drinking water needs.
- deposition
- The settling of sediment onto the bed, caused by decreased shear stress.
- disaggregation
- The process of floc breakup, where flocs are torn due to turbulence or collision.
- dispersive flux
- The portion of flux contributed by the correlated fluctuations of water motion and concentration of a substance.
- ebb
- The tidal water movement from landward to seaward, or out towards the ocean.
- erosion
- The mobilization of sediment from the bed, caused by increased shear stress. Conceptually the same as resuspension, but usually used in a different context.
- estuarine turbidity maximum (ETM)
- A zone of locally increased sediment concentrations, where some organisms may accumulate due to high nutrient concentrations. Because of high SSC, contaminant concentrations may also be high. The ETM can be formed by near-bed convergence of currents, tidal asymmetry, and cyclical transport of sediments, among other mechanisms.
- estuary
- Zone of mixing of saline and freshwater, though some estuaries may have no freshwater input depending on the season. Research in estuaries is difficult due to the dynamic mixing on multiple timescales. Estuaries are important to navigation (ports), recreation (boating, fishing), ecology (tidal marshes, juvenile fish), and contaminant transport (mercury, PCBs).
- extracellular polymeric secretions (eps)
- Secretions by organisms that may help sediment particles to stick together.
- floc
- A sediment aggregate that is composed of individual clay/silt particles, EPS, and interstitial water.
- flocculation (aggregation)
- The process of floc formation, which can occur when flocs collide with other flocs, and adhere to each other.
- flood
- When referring tides, the water movement from the ocean landward. When referring to freshwater flow, the episodic transport of water from watershed to ocean.
- flux
- The mass of a substance moving through a point or cross-section per unit time (like kilograms per second of sediment moving through a river).
- geomorphology
- The shape of any landform, submerged or exposed.
- gravitational circulation
- Circulation in estuaries caused by net freshwater outflow at the water surface and net saltwater inflow near the bed. The strength of circulation varies with tidal energy, freshwater flow, and water depth. It is capable of transporting sediment and salt upstream over prolonged periods.
- high tide
- The time of maximum water level at a given location.
- hydraulic mining
- The use of high-powered hoses to erode sediment from hillsides. Popular during the Gold Rush in California, the practice released large volumes of sediment to the San Francisco Bay-Delta.
- hydrodynamics
- The study of water physics, in any water body. Observations and modeling of water velocity, depth, waves, turbulence, and stratification are typical hydrodynamic studies.
- intertidal
- Portions of the estuary that are exposed at low-tide, and submerged at high-tide.
- low tide
- The time of minimum water level at a given location.
- marsh
- See wetland
- mud
- General name for clay and silt dominated sediment beds, that may include biological organisms.
- mudflat
- An area near the estuary fringes that consists of mud.
- neap tide
- Time of small tidal range, occurring about every 14 days in San Francisco Bay. Velocities are usually minimized during this time.
- nephelometric turbidity units
- Measure of turbidity.
- optical backscatter
- Method of light scattering used to estimate the concentration of sediment particles.
- primary particle
- The individual particle that may be incorporated into a floc.
- resuspension
- The mobilization of recently deposited sediment from the bed, caused by increased shear stress.
- salinity
- Non-dimensional measure of salt concentration in water. Ocean salinity is typically near 35, while freshwater salinity is by definition zero.
- sand
- Sediment particles size greater than 0.074 mm and less than 4.75 mm. Above 4.75 mm are gravel, cobble, and boulders.
- sea level rise
- The net trend of increasing ocean levels, confirmed at several sites worldwide. Sea level rise is caused by two processes: expansion of water due to increased temperature, and the melting of icecaps and other permanent ice features. At San Francisco, sea level rise has increased at about 2 mm/yr since 1900.
- sediment budget
- The net amount of sediment imported, exported, and stored within a water body.
- sediment
- General term for clay, silt, and sand particles that are submerged in a water body. Sediment may be associated with biological secretions (EPS) or hydrophobic contaminants.
- shear stress
- The force exerted by water velocity on a surface.
- silt
- Sediment particles sized less than 0.074 mm.
- slack
- The period of minimum water movement between flood and ebb. Sediment deposition is maximized during slack tide.
- snowpack
- The seasonal storage of snow in mountains that melts in the spring to provide freshwater runoff. California's water supply is heavily dependent on snowpack and snowmelt.
- spring tide
- Time of large tidal range, occurring about every 14 days in San Francisco Bay. Velocities are usually maximized during this time.
- suspended-sediment concentration (SSC)
- Dry mass of sediment in a given volume of water, usually expressed in milligrams per liter (mg/L) or kilograms per cubic meter (kg/m3).
- Stokes drift flux
- The portion of flux contributed by the correlated fluctuations of water motion and cross-sectional area.
- stratification
- Vertical layering of water masses, caused by density differences. Even in tidally mixed water bodies, stratification can be extremely strong. Turbulence works to break down stratification, which is strongest during neap tides when tidal mixing is weak.
- subtidal
- Any process that operates on timescales longer than the tides, such as wind forcing, or barometric pressure changes. Also refers to portions of the estuary that are never exposed at low-tide.
- tidal
- Any process related to the motions of the tides, caused by astronomical forcing (moon, sun, earth rotation). The "tidal" timescale refers to periods less than 12.5 hours, the time it takes to complete a tidal cycle (flood, ebb).
- turbidity
- Surrogate measurement for SSC, usually made with a nephelometric turbidity sensor. Light is emitted from one source on the sensor, and received on another. The turbidity is related to the amount of light received on the receptor.
- turbulence
- A feature of water movement recognized by rapid spatial and temporal variability. The opposite of laminar flow, where water movement is controlled and spatially/temporally ordered.
- USGS
- United State Geological Survey, a non-regulatory, scientific body under the Department of the Interior. The USGS is divided into the Geologic Division, Water Resources Division, Biological Resources Division, Geography Division, and the newly created Geospatial Program. This project is under the Water Resources Division.
- water year
- The year between October 1 and September 30. In California, the start of the water year coincides with the end of the dry season.
- watershed
- The area of land which channels water, sediment, nutrients, and contaminants down to the estuary and ocean.
- wetland
- A vegetated area typically inundated with water on various timescales. A tidal wetland may be inundated with every high tide, or only on the highest tides, depending on the elevation.
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