Soil erosion and sedimentation are naturally occurring processes.In fact, a certain amount of erosion is healthy for ecosystems,but too much erosion can cause problems. The watersheds in NapaCounty naturally produce relatively high amounts of sedimentowing to climate, topography, geology and soil conditions.However, this natural production has been accelerated by humanactivities over the past 150 years. Soil erosion and sedimentationare among the most serious threats to the long term healthof Napa County’s watersheds.

Understanding Napa County Watersheds

Soil Erosion and Sedimentation

Napa County Resource Conservation District

Why Should WWhy Should WWhy Should WWhy Should WWhy Should We Bee Bee Bee Bee BeConcerned About ErConcerned About ErConcerned About ErConcerned About ErConcerned About Erosion?osion?osion?osion?osion? Erosion removes topsoil and reduces soil fertility

A loss of topsoil, which often has important minerals, nutrient-rich soil organic matter, and high amounts of biological activity,creates a less favorable environment for plant growth. In theNapa Valley, where the rich topsoil is the source of its fame,erosion is a major concern.

Soil removed by erosion often ends up in waterwaysExcess sediment in streams, rivers, ditches, and reservoirs,reduces their capacity to store water resulting in more frequentand severe flooding and increased property damage. Sedimentaccumulations also damage storm drain systems.

Excess sediment degrades water quality and aquatic habitatHabitat areas and food sources are destroyed when fine siltscover the sand and gravel streambed. Decreased clarity of thewater prevents sunlight from reaching plants resulting in a lossof aquatic plant communities. The result is a reduction in thenumber and variety of fish and other aquatic life.

Too much sediment threatens the Napa River systemIn 1990 the San Francisco Bay Regional Water Quality ControlBoard placed the Napa River and its tributaries on the state’s“List of Impaired Waterbodies.” The listing wasmade in response to concerns that excess sediment wasdamaging habitat for steelhead trout, Chinook salmon, and otherthreatened species whose populations have declined substantiallyin recent decades. The list, which is a requirement of the federalClean Water Act, identifies waterbodies that do not meet waterquality standards and are not supporting their beneficial uses.Local agencies, landowners, and citizen groups are workingtogether to examine the problem, identify solutions, andultimately get the Napa River and its tributaries delisted.

Erosion is the physical wearingaway of the earth’s surface by the

agents of wind, ice, water, ormovement in response to gravity.Surface soil material is removed in

the process.

The severity of erosion depends ona combination of many factors,

including the amount and intensityof precipitation, the texture of thesoil, the steepness of the slope, and

the amount of ground cover.

Erosion is often quantified in termsof tons of soil loss per acre per

year. A ton of soil may be visualizedas the thickness of a sheet of paper

spread over an acre.

What is Erosion?What is Erosion?What is Erosion?What is Erosion?What is Erosion?

Once soil material is broken free, itmay be carried away and deposited

elsewhere. Sediment is solidmaterial that is or has been

transported from its site of originby air, water, gravity, or ice.

The process of depositingeroded material is known as

sedimentation. Some indications ofsedimentation are mud in the streetaround storm drains and decreasedwater holding capacity of a stream,

reservoir, or pond.

What isWhat isWhat isWhat isWhat isSedimentation?Sedimentation?Sedimentation?Sedimentation?Sedimentation?

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Understanding Napa County Watersheds Soil Erosion and Sedimentation

Napa County Resource Conservation District

Splash erosion occurs on the surface of thesoil, when particles are displaced by the forceof falling raindrops.

Sheet erosion occurs when rain is fallingfaster than soil can absorb it, causing water tocollect and flow over the ground. Particles thatwere detached by splash erosion can then becarried away by the surface water.

Splash and sheet erosion are nearlyimperceptible to the untrained eye. Leftuntreated, it slowly and insidiously wearsaway topsoil, also known as the “A” horizon.In Napa County hillsides the “A” horizon isonly 6 to 8 inches thick. A hillside without anadequate ground cover, for instance, is subjectto an entire loss of precious topsoil within 30to 40 years. Most soil scientists agree that ittakes at least 200 years to regenerate a mereinch of lost topsoil. Bare, newly disturbedsoils may suffer sheet erosion at rates of up to15 tons per acre per year without visualeffects. Displaced particles can move longdistances from a site or re-deposit nearby,depending on rainfall intensity and duration,topography, and the density of the particles.

The best defense against excessive sheeterosion is good vegetative cover. Onvineyards, cover crops are effective forpreventing sheet erosion. Cover cropestablishment is frequently enhanced byapplications of straw mulch, erosion controlblankets, and/or irrigation to germinate seedsprior to the onset of heavy winter rains.

TTTTTypes of Soil Erypes of Soil Erypes of Soil Erypes of Soil Erypes of Soil Erosionosionosionosionosion

Rill Rill Rill Rill Rill andandandandand Gully Er Gully Er Gully Er Gully Er Gully Erosionosionosionosionosion

Rill and gully erosion occurswhen sheet flows build velocityover unprotected soil and beginto establish paths. This type oferosion is more visuallynoticeable than sheet erosion.

Rill erosion results whensurface runoff concentrates andforms small, yet well-definedchannels, called rills. Rillsusually become apparent whenthe rate of soil loss exceeds 20tons per acre per year.

Gullies are formedwhen rills becomedeeper channels. Soilis rapidly removedby water gushingover the “headcut,”or uphill end of thegully. This rush ofwater scours thegully’s bottom, andremoves soilmaterial that hasslumped from the gully’s sidewalls. The slope at the headcutis nearly vertical, causing the runoff flowing over it to behighly erosive so that the gully advances upslope. Gullyerosion may lead to soil loss in excess of 300 tons per acreper year.

Rill and gully erosion are best prevented by minimizing sheetflows with good vegetative cover. Where topography andslope make concentration of sheet flow inevitable, flows canbe diverted to gently sloping earthen, grass- or rock-linedditches or protected outfalls with underground pipes. Suitableoutfalls may include well-vegetated natural waterways orfilter strips, rock energy dissipators, level spreaders, anddetention basins. Repair of existing gullies can beaccomplished with either hard armor, grade controlstructures, or bio-engineered installations such as willowrevetments and brush wattles. Restoration of native plantcommunities will frequently help prevent recurring problemsin critical or badly-damaged areas.

Gully erosion

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Rill erosion

Splash Splash Splash Splash Splash andandandandand Sheet Er Sheet Er Sheet Er Sheet Er Sheet ErosionosionosionosionosionThe Most Serious FThe Most Serious FThe Most Serious FThe Most Serious FThe Most Serious Formormormormorm

of Erosion in Napa Countyof Erosion in Napa Countyof Erosion in Napa Countyof Erosion in Napa Countyof Erosion in Napa County

Understanding Napa County Watersheds Soil Erosion and Sedimentation

Napa County Resource Conservation District

Mass WMass WMass WMass WMass WastingastingastingastingastingGravity is a constantforce, always pullingtowards the center ofthe Earth. This tirelessforce moves materialdownhill in a processknown as mass wasting.Any downslopemovement of rockand sediment is often

referred to in general terms as a landslide. However, masswasting covers many different phenomena, some of which arequite slow. These include falls, slumps, slides, and flows.

Mass wasting occurs when certain factors cause a change innormal conditions. A natural slope will try to reach a balancedequilibrium. If the conditions change, then this balance may beupset, and a sudden mass movement may take place. The factorswhich most influence stability are:

gravitythe shape and steepness of the slope.water, which weakens the cohesive strength of certainsaturated soils.sensitive soils, or materials which are particularlysusceptable to sliding. Soils most commonly affected arethose formed in sandstones, shales and serpentine rock.triggering events, or natural events like earthquakes andstorms.

In Napa County, mass wasting is common in the southern hills(Jameson Canyon, Carneros), eastern hills surrounding Woodenand Gordon Valleys, and Mayacamas Mountains (Mt. Veeder,Spring Mountain, Bear Creek Canyon) due to the inherentgeologic instability of soils. This susceptibility to mass wastingis sometimes exacerbated by tree removal, overgrazing andgrading. A common, very expensive mistake is to grade away thebase of a temporarily stable slide with a road cut, destabilizinga large mass uphill. Once activated, a slide can be very difficult torepair and stabilize.

An active slide is not only problematic for property owners, but, as a source of excess sediment inwaterways, it is also frequently a serious water quality threat. Thus, it is very important to be able torecognize ancient, dormant, and active slides, and to avoid them when possible. The County of Napamaintains geologic hazard maps which are helpful in identifying specific significant landforms that maybe unstable. Consultation with a geotechnical specialist or soils engineer is highly recommended whencontemplating any potentially invasive land use or development in an area subject to mass wasting.

A slump, the mostcommon type ofmass wasting inNapa County,involves the rotationalmovement of soil orother material alonga curved surface.

A fall is a sudden,nearly verticalmovement ofrock or soil. Forexample from anoverhanging cliff.

A flow is themovement ofwell-mixed,water saturatedsoil and rock.

A slide is similar toa slump except thematerial does notrotate. Block slidesare coherent blocksmoving down thehill side, while adebris slide is ajumble of material.

Diagrams created by Dr. Bruce Railsback, GeologyDept., University of Georgia

TTTTTypes of Mass Wypes of Mass Wypes of Mass Wypes of Mass Wypes of Mass Wastingastingastingastingasting

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Understanding Napa County Watersheds Soil Erosion and Sedimentation

For more information about erosion, practices tominimize erosion, and techniques used to restore

sites that are eroding, call or visit the:

Natural Resources Conservation Service (NRCS)or

Napa County Resource Conservation District(RCD)

1303 Jefferson St, Suite 500B, Napa, CA707/252-4188

A great deal of the NRCS and RCD energy is devotedto recognizing and mitigating accelerated, human-

induced soil erosion and sedimentation. Technical andfinancial assistance may be available.

Napa County Resource Conservation District

Streambank ErStreambank ErStreambank ErStreambank ErStreambank ErosionosionosionosionosionStreambank erosion is a natural phenomenon. Creeks and rivers inequilibrium are dynamic systems: moving and depositing predictableamounts of sediment and shifting, eroding, and meandering withindefinable, usually well-vegetated riparian belts. A river’s equilibriumis upset when changing land uses, such as road construction andurban and agricultural development, modify the hydrology of thewatershed by reducing infiltration rates and increasing runoff ratesand peak storm flows. These increased flow velocities in turn increasethe erosive and destabilizing forces on streambanks.

Incursion of intensive land uses onto floodplains and other portions of the riparian belt, combined withincreased flow velocity, frequently lead to property damage. However, confining the river with levees to protectproperty within the flood plain or riparian belt, thus isolating the river from its floodplain, can exacerbateflooding, bank failures, and excess sedimentation downstream. Bank failures can be, and frequently are,addressed on a piecemeal, site-by-site basis with riprap emplacement or other hard-armor structures. These

measures are frequently ineffective or short-lived, and nearly alwayshave unintended side effects—both upstream and downstream.Revegetation of the streambanks using bio-engineered installationssuch as willow revetments are often more effective.

Long-term solutions to streambank instability must focus onre-establishing the river’s equilibrium, by modifying land use practiceswithin the watershed to more closely emulate the natural hydrology,and by allowing the river a sufficient riparian belt-width in which toresume normal fluvial function.

Sources

This fact sheet was prepared by the Napa County Resource Conservation District with assistance from theNatural Resources Conservation Service. Funded through Napa County’s Watershed Education Program coordinated by the

Conservation Division of the Napa County Conservation, Development and Planning Department.

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Willow revetment wall along the bank of Selby Creek.

Franklin Soil and Water Conservation District and USDANatural Resources Conservation Service. 2004. Erosion &Sedimentation Fact Sheet. www.franklin.swcd.org.

Mount, Jeffery F. 1995. California Rivers and Streams: TheConflict Between Fluvial Process and Land Use. Berkeley, CA:University of California Press.

Napa County Resource Conservation District. 1994. NapaRiver Watershed Owner’s Manual: An Integrated ResourceManagement Plan. This manual is a collection of naturalresource management reccommendations developed by theRCD with assistance from private citizens, local interestgroups, and federal, state, and local government agencies.It can be purchased at the RCD/NRCS office. You canalso view sections of it on the RCD’s website atnaparcd.org/ownermanual.htm.

USDA Natural Resources Conservation Service. 1996. SoilQuality Information Sheet: Soil Erosion. soils.usda.gov/sqi.