effects of tide gates web - oregon

by Guillermo R. Giannico and Jon A. Souderby Guillermo R. Giannico and Jon A. Souder

The Effects of Tide Gateson Estuarine Habitats and Migratory Fish

The Effects of Tide Gateson Estuarine Habitats and Migratory Fish

Guillermo R. Giannicois on the faculty of theDepartment ofFisheries and Wildlifeat Oregon StateUniversity, and Jon A.Souder is executivedirector of the CoosWatershed Association.

D ikes and tide gates have been usedworldwide for several centuries todrain wetlands, both in estuaries

and in the lower sections of rivers, which areinfluenced by tides. Wetland draining hasbeen carried out either to convert lands intoagricultural use, to control populations ofmosquitoes and other insects, or to allowurban development on low-lying coastal zones(Daiber 1986; Middleton 1999; Doody 2001).In the Pacific Northwest region of NorthAmerica, the draining of estuarine wetlandsbegan approximately two hundred years ago(Dahl 1990). Tidal marshes close to seaportsand urban centers have been particularlyvulnerable to conversion, with losses of 50 to90 percent reported for many estuaries inOregon and Washington (NRC 1996). Manyof these marshes have been isolated from theadjacent estuaries by dikes (Frenkel andMorlan 1991) and in some cases completely orpartly filled in to accommodate a variety ofland uses (for example, agricultural, recre-ational, residential, and industrial). In areaslike Coos Bay, Oregon, almost 90 percent oftidal marshes have been permanently lost todikes and landfills (Schultz 1990), and in partsof Puget Sound, Washington, over 95 percentof tidal wetlands have been lost (Gregory andBisson 1997).

Dikes are elevated earthen embankmentsraised along tidally influenced channels inestuaries and coastal sections of rivers to keeplow-lying lands from being flooded duringhigh tides. Structures known as flood boxes, ortide boxes, are installed in dikes to control the

flow of upland water through creeks orsloughs into estuaries or rivers. A flood boxmight be as simple as a single culvert runningthrough a dike wall or as complex as a small,bridge-sized, concrete structure that includestwo or more culverts, deflection wing walls,and upstream and downstream pilings (seefigures 1a and 1b). In all cases, doors or lidsare attached to the discharge ends of theculverts to control water flow. These doorsare commonly referred to as tide gates, or flapgates (figure 1b). Tide gates close duringincoming (flood) tides to prevent tidal watersfrom moving upland. They open duringoutgoing (ebb) tides to allow upland water toflow through the culvert and into the receiv-ing body of water. Figure 2 illustrates theentire gate cycle as water levels change.

Tide gates tend to be effective at main-taining low water levels on the upland side ofdikes. Unfortunately, by altering water flowthey have some undesirable side effects thatcan be classified into three main—butinterrelated—categories: physical, chemical,and biological.

The physical effects of tide gates includeelimination of upland tidal flooding andchanges in the velocity, turbulence, andpattern of freshwater discharge that fluctu-ates between water stagnation and flushingflows. In turn, these changes in the circula-tion of water between both sides of a dikecause alterations in water temperatures, soilmoisture content, sediment transport, andchannel morphology (Vranken and Oenema1990; Charland 1998).

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2 The Effects of Tide Gates in Estuarine Habitats and Migratory Fish

The chemical effects of tidegates consist of upstream increases inwater nutrient concentration,turbidity, and heavy metal suspensionand reductions in dissolved oxygenand pH (that is, water alkalinity)(Portnoy 1987; Vranken andOenema 1990). Soil salinity is alsoreduced because tide gates preventbrackish tidewaters from reachingpast dikes, and the freshwater that isallowed to drain toward the estuaryremoves salts from soils over time(Dreyer and Niering 1995).

The biological effects of tidegates come in the form of obstruc-tion to fish migration, changes to thecomposition of aquatic plants, andpulses of coliform bacteria intoestuarine waters during low tides(Eliasen 1988; Charland 2001).

Although the tide gate itself iswhat stops water and fish movement,it is important to recognize that thedesign of the entire flood box affectsthe circulation of water and fish

passage. Thus, the effect that floodboxes have on the aquatic environ-ment depends on a combination ofthings: culvert diameter relative tothe upstream channel; culvert surfaceroughness; channel bottom charac-teristics both upstream and down-stream from the flood box; culvertbottom (also known as invert or sill)elevation relative to the channelbottom and to tide levels; type of tidegate; wing wall orientation; presenceof pilings in the channel; and otherfeatures of the flood box (figures 1aand 1b).

There are many different typesof tide gates, but the traditionalconfigurations have either a heavy lid(made of treated wood) suspendedfrom the top of the culvert by eithera bar with hinges or chains (figure3a), or a metal lid (usually of castiron or steel) double hinged fromabove (figure 3b). Alternative designsinclude side-hinged gates (figures4a–c) and top-hinged gates with

small “pet doors” (figure 5)(Charland 2001). Tide gates openand close because of water leveldifferences between the downstreamand the upstream sides of the gate.Water level differences are caused by(1) tidal cycles (and magnitudes),(2) inflow into the reservoir pool thatforms on the upland side, and (3) theextent to which this reservoir poolhas been drained during previousgate-opening cycles.

The amount of water leveldifference required to open a tidegate is determined by the “effectiveweight” of the gate. For top-hingedgates, both the amount of time andthe degree of opening are functionsof the weight and area of the gateand the pressure force generated bythe difference in water level betweenboth sides of the gate (Raemy andHager 1998). Thus, the difference inwater level needed to open a gate isgreater the larger the gate and themore it weighs (USACE 2001).Conversely, for side-hinged gates,the effective weight of the gate is afunction of the resistance of itshinges and the degree to which thedoor is tilted downward. In general,and because of the opening forcecaused by their tilt, side-hinged gatesopen wider and for longer periodswith less upland water pressure thanequivalently sized top-hinged gates.Top-hinged tide gates may have “petdoors” that are hinged at the top,side, or bottom. These smaller doorsremain open for longer periods thanthe large gates they are part of and,as a result, they facilitate fish passage(Charland 2001).

Unfortunately, tide gates caneasily be jammed by floating piecesof wood, and, over time, they tend tohang twisted from their hinges. Thischanges the way the gates operateand creates problems in terms ofboth fish passage and flood control.Regular inspections are recom-mended to avoid this type of problem.

a

b

Figure 1. Flood box (culvert with tide gate) installed in dike’s wall: (a) top view;(b) side view.

3The Effects of Tide Gates in Estuarine Habitats and Migratory Fish

Physical Effectsof Tide Gates

Changes to Channel MorphologyChannel morphology is altered

by tide gates in two ways. First,upstream scour tends to form aninlet pool, and the water jet throughthe culvert forms a deep scour poolon the outlet end. Second, if theflood box is replaced and the bottomof the culvert is set at a lower level,then upstream erosion of the accu-mulated sediments could result inchanges to the channel morphology.

Changes to Water TemperatureWater temperature not only

plays a critical role in affecting thespeed at which many chemicalprocesses occur (Richardson andVepraskas 2001), but it is alsoextremely important in determiningthe suitability of habitat for differentaquatic organisms. Water tempera-ture criteria are established under theClean Water Act for salmon andtrout spawning (55˚F) and nursery(64˚F) habitats (ODEQ). In Oregonand Washington, limiting tempera-tures in coastal streams typicallyaffects summer nursery habitat. This

is because water temperatures duringthe spawning periods are below thespawning temperature standard.Because tide gates cause freshwaterstagnation and restrict tidal inflow,they tend to increase upstream watertemperatures. The increased periodof water circulation allowed by side-hinged gates may result in lowerwater temperatures by reducingfreshwater stagnation and augment-ing cooler estuarine inflow.

Figure 2. Side view of tide gate: (a) closing, as upstream water level drops and tide level (see arrow) begins to increase; (b) closed, astidal water level gets higher (see arrow) than upstream freshwater level; (c) opening, as tidal water level becomes lower (see arrow)than the freshwater level inside culvert; (d) open, during low tide (see arrow) before upstream water level begins to decline.

a b

c d


Chemical Effectsof Tide Gates

Changes to WaterSalinity

Salinity in wetlandsoils determines theiroxidation-reductionpotential, ultimatelycontrolling soil pH.Soil pH, in turn,affects the sequestra-tion and liberation ofheavy metals.

Water salinity inestuaries varies dailyand seasonally. Dailyvariations are influ-enced by tides, aswater with higher

salinity flows from the mouth of theestuary inland during flood tide andback toward the ocean during ebbtide. The extent of brackish waterupstream is determined by the

Figure 3. Top-hingedtide gates (a) suspendedwith chains, (b) held inplace by metal arms, and(c) illustrated.

amount of tidal exchange (that is, thedifference between high and lowtides for any specific cycle) and bythe amount of freshwater enteringthe estuary from tributary streams.In the Pacific Northwest, freshwaterflow into estuaries varies by season,with high flows during the winterand spring and low flows during thesummer and fall. Because salt waterhas a greater density than freshwater,it tends to occupy the lower portionof the water column. Thus, incomingsalt water dives below the outgoingfreshwater and creates a wedge thatmoves along the bottom. The sizeand shape of this wedge depend onthe volume of freshwater that entersthe estuary, the shape of the estuary,and the prevailing coastal currents.

A tide gate prevents the floodingof upland channels by brackish water.As a result, a dramatic difference insalinity exists between one side of thegate and the other. When the gateopens, pooled freshwater moves into

b

a

DTidal Level

z

Tidal level

c


DTidal Level

z

Tidal level

the estuarine channel, creating atongue of fresher water that, throughturbulence, mixes as it moves downthe estuary. The speed of the salinitymixing—and the extent of thefreshwater tongue—is related to thetype and size of the gate, the amountof freshwater pooled upstream, andthe relative difference in salinitybetween fresh and brackish water inthe area (Jay and Kukulka 2003).

Over time, all tide gates begin toleak. Leaks occur if the seal betweenthe gate and the supporting structure

is uneven or if material is caughtwhen the gate closes. Leaks are alsocommon when culverts corrode orcrack upstream from the gate. Inaddition, leaks may happen when saltwater percolates through a dike’s fillmaterial because of the hydraulicpressure caused by high tides.Whenever brackish water enters theupland side of a dike, it occupies thebottom layer of water in upstreampools. This water may form residualpools of brackish water if the fresh-water outflow when a tide gate opens

is insufficient to create the velocityneeded to mix both layers of water.

Changes to Heavy-Metal Concentrationin Water

Soils in estuarine marshes arenaturally anaerobic (that is, they lackoxygen). When the operation of tidegates begins to lower the salinity ofsoils on the upland side of dikes andperiodically desiccate them, thesesoils become exposed to air, and avariety of aerobic (that is, oxygen-

Figure 4. (a) On the left is a side-hinged round gateand on the right, in the same photo, is a top-hingedgate with a mitigator fish-passage device; (b) a side-hinged rectangular gate; and (c) (right) diagram ofan open side-hinged gate.

a b

c


driven) processes begin in them(Richardson and Vepraskas 2001). Ifthis occurs, immobilized, reducedsulfides combined with the soil ironare oxidized and converted intosulfates and sulfuric acid. Thesecompounds make the soil acid (thatis, lower its pH) (Anisfeld and Benoit1997; Portnoy and Giblin 1997), andthis acidification can cause the heavymetals in the soil (such as lead,copper, silver, and cadmium) to bereleased into the water (Anisfeld andBenoit 1997).

Biological Effectsof Tide Gates

Although tidal marshes providecritical habitat to many aquaticorganisms (Healey 1982; Simenstadet al. 1982; Shreffler et al. 1990 and1992), store upland sediments(Vranken and Oenema 1990;Portnoy 1999), and regulate flood-waters (Turner and Lewis 1997), theconsequences of their alteration bydiking and filling has not attractedthe same attention from regulatoryagencies as have other developmentprojects. Whereas dams, roads,

culverts, and water diversion projectshave been the focus of many impactstudies, and their construction oroperations are tightly regulated, theimpacts of dikes and tide gates onmigratory fishes have receivedrelatively little attention. Onlyrecently have the effects of dikes andtide gates on salmon and troutmovement and habitat access at-tracted the interest of agencies andwatershed councils as salmonidsbegan to decline in abundance(Nehlsen et al. 1991).

In the particular case of anadro-mous salmon and trout (whichmigrate from freshwater environ-ments to the sea and back), tide gatesare alleged to negatively affect them,not only by preventing their migra-tion but also by deteriorating thequality and connectivity of theirhabitats. The notion that tide gatesinterfere with fish migration hasencouraged the development of“fish-friendly” gate designs (Eliasen1988; Thomson and Associates2000). Unfortunately, studies on theeffectiveness of such alternativedesigns have not been carried out byindependent research institutions,and the limited information available

on their effects comes entirely fromtide gate manufacturing companies.

Tide Gates as Physical Barriersto Fish Passage

Two factors influence the degreethat a tide gate represents a physicalbarrier for fish. One is the length oftime the gate is closed and the other,the size of the opening.

Any tide gate represents a totalbarrier to fish passage during thetime it remains completely closed.The length of this period depends onthe magnitude of the tidal exchange(the difference between high and lowtides), the water inflow into theupstream pool between openingcycles, and the degree to which thispool emptied during the previouscycle. Under normal conditions, top-hinged tide gates will not open untilthe water level inside the culvert ishigher than the water level on thedownstream side. These gates willclose only when the water level onthe downstream side is equal to orhigher than the water level inside theculvert. Because tidal cycles areapproximately 12 hours long andtides flood (flow in) about half of thattime and ebb (flow out) the otherhalf, top-hinged tide gates areexpected to remain closed at least 50percent of the time (that is, 6 hourswithin each cycle, assuming theupstream pool empties completelyduring each cycle and the gate closesby the combined effects of its ownweight and slack tide). However,depending on how they are installedand the characteristics of the areathey drain, gates may remainedclosed for longer periods. For ex-ample, Scalisi (2001) reported thatduring February 2001 the tide gate inLarson Slough, Coos Estuary,Oregon, opened 4 hours after thebeginning of ebb tides and closed atslack tide, thus, representing a totalbarrier to fish passage 75 percent ofthe time.

Tidal Level

Figure 5. A top-hinged gate with a small “pet door” attached to floater.


Because all tide gates block fishpassage during all or most incomingtides, there is no such thing as a“fish-friendly” tide gate, only “fish-friendlier” ones. Compared to themost restrictive gates, a “fish-friendlier” installation should have agate that opens wider and for longerperiods of time, creates less watervelocity and turbulence, and providesa gradual transition between freshand salt water, with salinity refugiaavailable for juvenile fish.

“Fish-friendlier” tide gatedesigns employ three mechanisms toincrease the physical passage period.First, since gate weight determinesthe amount of water level differencerequired to open the gate and keep itopen, the use of lighter materialssuch as aluminum, fiberglass, andplastics in both top-hinged and side-hinged gates will increase the timethey remain open.

Second, because the “effective”weight (that is, the gate-closingforce) of side-hinged gates is lowerthan that of top-hinged gates, side-hinged gates will open earlier andclose later in a tidal cycle than top-hinged gates of equivalent size. Someside-hinged gates have been reportedto require only one inch of waterlevel difference to open up to 45˚(Scalisi 2001). Such an opening anglewith a small water level differencerepresents a significant improvementto fish passage when the width of theopening is factored in (one foot isconsidered the minimum for adultfish passage by the WashingtonDepartment of Fish and Wildlife).

A third mechanism employed toimprove gate fish passage wasdesigned by Leo Kuntz (personalcommunication). This mechanism isa float-activated cam, known as amitigator fish-passage device, that iswedged between the gate and themouth of the culvert to keep the gateopen during incoming tides (seeright tide gate in figure 4a). The

floats regulate the amount ofbackflow allowed upstream beforethe gate closes when the cam isreleased by the upward movement ofthe floats under a rising tide. Thetidal level at which the cam isreleased is adjustable to regulateupstream backwatering.

Tide gates not only constitutedirect physical barriers to fishpassage, but also create indirectobstacles to fish in the form ofelevated water velocities and turbu-lence. Velocity criteria establishedfor fish passage in culverts are similarto those used for tide gates. InOregon, the water velocities recom-mended by the Department of Fishand Wildlife are 5 feet per second foradult salmonids in culverts 60 to 100feet long, and 2 feet per second forjuvenile salmonids (Robison et al.1999).

Average water velocities throughtide gates are a function of twothings: (1) the upstream-downstreamwater level difference, which to adegree varies through the tidal cyclewith different opening angles; and(2) the width of the opening, whichalso varies through the tidal cyclewith different opening angles andwhich is influenced by the resistanceof the gate to opening, depending onits weight and design. Water veloci-ties through side-hinged gates arelower than through top-hinged gatesof similar size and weight becauseless force is required to keep side-hinged gates open. Also, lighteraluminum gates require less force toopen, and as a result, velocitiesthrough their openings are lowerthan with steel or cast-iron gates ofcomparable size.

Water turbulence results fromthe effects of shear forces caused bydrag in water velocity at the edges ofchannels, through obstructions, orwhere differences in viscosity occur(Goldstein 1965). The width and theshape of the channel, its substrate

composition, bank roughness, andprotuberances in the channel allreduce the velocity of the layer ofwater that is in contact with the wallsof the channel (or culvert, in the caseof a flood box) in relation to theaverage velocity of the rest of thewater column. When a criticalthreshold in the velocity differencebetween these layers of water isreached, turbulent flow results.

Turbulence and the associatedbubbling of air in water causevibration and noise that, dependingon their magnitude, may representobstacles to fish movement. Heavytop-hinged tide gates produce ahigh-velocity jet of water, called venacontracta (see figure 1b), whichcreates turbulence and bubbling(figure 3b) (Pethick and Harrison1981). This water jet is caused by thecombined effects of the upstream-downstream water level differences,the tendency of the gate to close bythe effect of its own weight (orrestorative force), and the size of thegate.

On the basis of our observations,turbulence seems to be lower with“fish-friendlier” side-hinged gates(figure 4b) than with top-hingedones. Because the forces that tend tokeep the gate closed are lower inboth side-hinged and light-weighttide gates, the jet of water and itsresulting water turbulence andbubbling are practically eliminated.

Effects of Tide Gateson Salmon Nursery Habitats

Estuaries play a critical role injuvenile salmonid survival during thetransition from fresh to salt water(Pearcy 1992). Estuarine habitatsprovide juvenile salmon with aproductive feeding area, a refugefrom marine predators, and a transi-tional zone for gradual acclimationto salt water (Thorpe 1994).

Because tide gates interfere withwater flow in the boundary zone


between estuaries and streams, theynegatively affect the coastal marshhabitats of juvenile salmon. They dothis not only by altering waterquality and channel morphology, butalso by changing the species ofaquatic plants and invertebrates (forexample, insects and crustaceans)that juvenile fish rely on for coverand food.

Literature CitedAnisfeld, S. C., and G. Benoit. 1997.

Impacts of flow restriction on saltmarshes: an instance of acidification.Environmental Science & Technology31:1650–1657.

Charland, J. W. 1998. Tide gate modifica-tions for fish passage and water qualityenhancement. Tillamook Bay NationalEstuary Project, Garibaldi, Oregon.

_________. 2001. Alternative tide gatedesigns for habitat enhancement andwater quality improvement. MarineEnvironment Engineering Technol-ogy Symposium, Alexandria, Virginia.

Dahl, T. E. 1990. Wetland losses in theUnited States: 1780s to 1980s. U.S.Department of the Interior, Fish andWildlife Service, Washington, D.C.

Daiber, F. C. 1986. Conservation of tidalmarshes. Van Nostrand Reinhold,New York, New York.

Doody, J. P. 2001. Coastal conservation andmanagement: an ecological perspective.Kluwer Academic Publishers, Boston,Massachusetts.

Dreyer, G. S., and W. A. Niering. 1995.Tidal marshes of Long Island Sound:ecology, history and restoration. Con-necticut College Arboretum Bulletin,Connecticut College Arboretum,New London, Connecticut 34:1–72.

Eliasen, R. 1988. Side hinged flap gates: analternative to salmonid migrationbarriers in flood drainage. Departmentof Fisheries and Oceans, Nanaimo,British Columbia.

Frenkel, R. E., and Morlan, J. C. 1991.Can we restore our saltmarshes?Lessons from the Salmon River,Oregon. Northwest EnvironmentalJournal 7:119–135.

Goldstein, S. (editor). 1965. Moderndevelopments in fluid dynamics: anaccount of theory and experiment relatingto boundary layers, turbulent motion andwakes. Vol. I. Dover, New York, NewYork.

Gregory, S. V., and P. A. Bisson. 1997.Degradation and loss of anadromoussalmonid habitat in the PacificNorthwest. Pages 277–314 in D. J.Stouder, P. A. Bisson, and R. J.Naiman, editors. Pacific salmon andtheir ecosystems: status and future options.Chapman and Hall, New York, NewYork.

Healey, M. C. 1982. Juvenile Pacificsalmon in estuaries: the life supportsystem. Pages 315–41 in Victor S.Kennedy, editor. Estuarine comparisons:proceedings of the sixth biennial interna-tional estuarine research conference,Gleneden Beach, Oregon, November 1–6,1981. Academic Press, New York.

Jay, D. A, and T. Kukulka. 2003. Revisingthe paradigm of tidal analysis—theuses of non-stationary data. OceanDynamics 42:1–16.

Kuntz, Leo. Personal communication.Nehalem Marine, 24755 Miami RiverRoad, Nehalem, Oregon 97131.

Middleton, B. 1999. Wetland restoration,flood pulsing, and distribution dynamics.John Wiley & Sons, New York, NewYork.

Nehlsen, W., J. E. Williams, and J. A.Lichatowich. 1991. Pacific salmon atthe crossroads: stocks at risk fromCalifornia, Oregon, Idaho, andWashington. Fisheries 16(2):4–21.

NRC (National Research Council). 1996.Upstream: salmon and society in thePacific Northwest. National AcademyPress, Washington, D.C.

ODEQ (Oregon Department of Envi-ronmental Quality). Chapter 340.Water pollution. Division 41. State-wide water quality management plan;beneficial uses, policies, standards, andtreatment criteria for Oregon. http://www.epa.gov/ost/standards/wqslibrary/or/or_10_wqs.pdf

Pearcy, W. G. 1992. Ocean ecology of NorthPacific salmonids. University ofWashington Press, Seattle.

Pethick, R. W., and A. J. M. Harrison.1981. The theoretical treatment of the

hydraulics of rectangular flap gates.Pages 247–254 in Proceedings, 19thcongress, International Association forHydraulic Research, New Delhi, India1–7 February 1981.

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Portnoy, J. W., C. T. Roman, and M. A.Soukup. 1987. Hydrologic andchemical impacts of diking anddrainage of a small estuary (Cape CodNational Seashore): effects on wildlifeand fisheries. Pages 254–65 in W. R.Whitman and W. H. Meredith,editors. Waterfowl and wetlandssymposium: proceedings of a symposiumon waterfowl and wetland managementin the coastal zone of the Atlantic flyway.Delaware Department of NaturalResources and EnvironmentalControl, Dover, Delaware.

Portnoy, J. W., and A. E. Giblin. 1997.Effects of historic tidal restrictions onsalt marsh sediment chemistry.Biogeochemistry 36:275–303.

Raemy, F., and W. H. Hager. 1998.Hydraulic level control by hinged flapgate. Proceedings Insitute of CivilEngineers, Water, Maritime, and Energy130:95–103.

Richardson, J. L., and M. J. Vepraskas(editors). 2001. Wetland soils: genesis,hydrology, landscapes, and classification.CRC Press, Boca Raton, Florida.

Robison, E. G., A. Mirati, and M. Allen.1999. Oregon road/stream crossingrestoration guide: spring 1999. Ad-vanced fish passage training version.Oregon Department of Forestry,Salem, Oregon.

Scalisi, M. 2001. Larson Slough tidegatestudy: an investigation of the hydro-dynamics across the Larson SloughInlet and water quality of LarsonSlough. Second Draft Report to theCoos Watershed Association.

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Shreffler, D. K., C. A. Simenstad, and R.M. Thom. 1992. Foraging by juvenilesalmon in a restored estuarinewetland. Estuaries 15:204–213.


Shreffler, D. K., C. A. Simentstad, andR. M. Thom. 1990. Temporaryresidence by juvenile salmon in arestored estuarine wetland. CanadianJournal of Fisheries and Aquatic Sciences47:2079–2084.

Simenstad, C. A., K. L. Fresh, and E. O.Salo. 1982. The role of Puget Soundand Washington coastal estuaries inthe life history of Pacific salmon: anunappreciated function. Pages 343–364 in V. S. Kennedy, editor. Estuarinecomparisons. Academic Press, NewYork, New York.

Thomson, A. R., and Associates. 2000.Flood boxes as fish migration barriers inlower mainland streams. Prepared forthe Ministry of Environment, Landsand Parks, Region 2, Vancouver, B.C.

Thorpe, J. E. 1994. Salmonid fishes andthe estuarine environment. Estuaries17:76–93.

Turner, R. E., and R. R. Lewis. 1997.Hydrologic restoration of coastalwetlands. Wetlands Ecology andManagement. Special issue: Hydro-logic restoration of coastal wetlands4:65–72.

USACE (U.S. Army Corps of Engi-neers). 2001. HEC-RAS river analysissystem—hydraulic reference manual.Version 3.0. U.S. Army Corps ofEngineers, Institute for WaterResources, Hydrologic EngineeringCenter, UC Davis, California.

Vranken, M., and O. Oenema. 1990.Effects of tide range alterations onsalt marsh sediments in the EasternScheldt, S. W. Netherlands.Hydrobiologia 195:13–20.

This research and publication were funded by the National Sea Grant CollegeProgram of the U.S. Department of Commerce’s National Oceanic andAtmospheric Administration, under NOAA grant number NA76RG0476(project number R/HBT-07-PD), and by appropriations made by the OregonState legislature. The views expressed herein do not necessarily reflect the viewsof any of those organizations.

Sea Grant is a unique partnership with public and private sectors, combiningresearch, education, and technology transfer for public service. This nationalnetwork of universities meets the changing environmental and economic needsof people in our coastal, ocean, and Great Lakes regions.

CreditsEditing and layout: Sandy RidlingtonIllustrations: Jon A. SouderPhotographs: Coos Watershed Association

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effects of tide gates web - oregon

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