Effects of Turbidity and

Hydrodynamics on

Distributions of Delta Smelt Peter E. Smith, USGS, ret.

Tues, Apr 17, 2012, 2012 CWEMF Annual Meeting

Lake Natoma Inn, Folsom, CA

Juvenile cod and mysid Juvenile Atlantic cod (Gadus morhua)

prey in turbid water in turbid water

A Few Comments About

Turbidity

• Turbidity (also referred to as water clarity) is an optical property of water that causes the scattering and absorption of light mainly by suspended sediment, dissolved organic matter, and plankton.

• In the Delta, turbidity is affected most by suspended sediment.

A Few Comments About

Turbidity (cont’d)

• Levels of turbidity can fluctuate with natural events such as large inflows, wind, human activity (dredging, for example), and local runoff from rain.

• In the Delta, turbidity levels are often highest during the “first flush” of winter.

Turbidity and Fish Behavior

• Behavioral rather than physiological effects in estuaries.

• Effects of turbidity on freshwater fishes is relatively well known, but little is know about the effects on marine and estuarine species. Among freshwater fishes there is fairly clear evidence that turbidity can play a significant role in distributions of fish (mostly diel horizontal migrations in shallow lakes).

Turbidity and Fish Behavior

(cont’d) • Reduced turbidity may lead to migration

(movement) to other habitats.

• The ability of both predator and prey to detect each other can be affected by turbidity.

• Turbidity can act as cover for small fish and reduce predation risk.

Turbidity and Fish Behavior

(cont’d) • In areas of low background turbidity, small

increases in turbidity can enhance feeding of larval fish by increasing prey contrast. (A similar visual effect is evident when viewing a nearly object in thick fog.)

• Turbidity should decrease feeding rates for piscivorous fish (fish that eat other fish), which feed on larger more visible prey and increase (or decrease less) the feeding rate for particle-feeding planktivorous fish (fish that eat plankton).

Contrast and Fog Photography

Contrast and Fog Photography

(cont’d)

The scattering of light that occurs in the atmosphere due to thick

fog gives nearby objects more contrast and distant objects less

contrast; nearby objects appear sharper and distant objects

appear lighter and less visible. The apparent depth between

near and far objects is enhanced.

Vernalis

Suspended Sediment

Turbidity

Long-Term

Monitoring

Stations

(since 1960)

(since Mar

1988)

Map of North Delta

Tract Franks

lu

Delta Cross Channel

Mokelumne River

boa

t

Slough Cache

Lib

ert

y I

s.

North Delta

Little Potato Slough

During the first flush, sediment load from the Sacramento River enters the central Delta from the mouth of the Mokelumne R. via Georgiana Sl.

Map of North Delta

Tract Franks

lu

Delta Cross Channel

Mokelumne River

boa

t

Slough Cache

Lib

ert

y I

s.

North Delta

Little Potato Slough

If the DCC is left open, 150 percent more sediment load is diverted.

Jan-Feb Mean turbidity Spring Midwater Trawl

January turbidity Spring Midwater Trawl

1993

1994

The difference

between a

wet and dry

winter can be

dramatic

We learned in 2010 that there can be significant turbidity

gradients in the south Delta during first flush

Tidally averaged

During first flush there can also be significant tidal variations

in turbidity

San Joaquin River at Prisoner’s Point

Turbidity and Delta Smelt

Probability of Catching Delta

Smelt

Turbidity vs Delta Smelt Salvage

Delta Smelt

1993-2005, Dec-Mar

Salvage, OMR Flows, and South Delta Turbidity

Note: Delta smelt population size, SJR flow, and XGEO also can affect the magnitude of winter salvage

Adult Delta Smelt Salvage and FMWT Index

Total Delta Smelt Adult Salvage (WY Dec-Mar)

Fall Midwater Trawl Index for Delta Smelt

Adult

Salvage,

In fish

Delta Smelt

FMWT

Delta Smelt

Index for

No

in

de

x

59,400 fish

Typical Change in Delta Smelt and Turbidity Distributions

from Fall to Winter during a POD Year

WY 2002

Date of SKT Survey 1

Jan 7-10, 2002 SKT Survey 1

Water Year 1981

8,630 cfs

5,410 cfs

729 tons/day

1,270 tons/day

Unusual conditions

during fall 1980 in

which the south

Delta is more turbid

than the Sacramento

River.

Fall 1980 Delta Smelt Map

zoomed in

End