Wetland Hydrology Chapter 2: Wetland Hydrology 2: Wetland Hydrology 11 wetland. The outflow of water…

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Chapter 2: Wetland Hydrology 9The term wetland hydrology generally refers to the inflow andoutflow of water through a wetland and its interaction with other sitefactors. Land is characterized as having wetland hydrology when,under normal circumstances, the land surface is either inundated orthe upper portion of the soil is saturated at a sufficient frequency andduration to create anaerobic conditions. The presence or absence ofwetland hydrology may be determined through the on-siteidentification of established field indicators. While field indicators ofwetland hydrology are at times difficult to identify, it is essential todetermine that the area is periodically inundated or has saturatedsoils in order for the area to be characterized as meeting the wetlanddefinition. For sites where there is a predominance of wetland plantspecies but there is no direct visible evidence that water is, or hasbeen, at or above the soil surface, Part 303 directs the MDEQ to usethe characteristics of soils to verify the presence or absence ofwetland hydrology. Soils and field indicators of wetland soils arediscussed in Chapter 4. If field indicators of hydrology are absent,such as in disturbed areas, evidence of hydrology may need to beestablished through the evaluation of recorded hydrologic data.A. Site Factors that Influence Wetland HydrologyWetland conditions occur where topographic and hydrogeologicconditions are favorable and a sufficient, long-term source of waterexists. Favorable topographic conditions refer generally to thepresence of land-surface depressions in the drainage basin. Thesedepressions may be located in upland areas, along hillsides wherethere may be a change in slope or geology, in floodplains of streamsor rivers, or along the margins of lakes. Geologic conditions whichmay be favorable for wetland development include areas that havefine textured surficial soils with low hydraulic conductivity andsufficient thickness to store water. Also, the presence of impermeablebedrock near the land surface may favor the development of wetlandhydrology.The development of wetland conditions requires a persistent, long-term source of water. Figure 2.1 shows the different sources of waterand possible outflows from a hypothetical wetland. The source ofwater may be precipitation (P) which falls directly on the wetland,surface water runoff during rainfall or snowmelt events within thecatchment area surrounding the wetland (surface water inflow, orSWI), periodic flooding caused by elevated water levels in nearbysurface water bodies (also SWI), groundwater inflow to the wetland(GWI), or a combination of any, or all, of these sources. Water maybe lost from a wetland by evaporation from standing water or2CHAPTERWetland Hydrology10 MDEQ WETLAND IDENTIFICATION MANUALsaturated soils (E),transpiration from plants(T), or surface water orgroundwater outflow(SWD or GWD).The development ofwetland conditionsdepends on a long-termbalance between waterinflow to the wetland andoutflow from the wetland.During dry climaticperiods, the rate of waterinflow to the wetland(precipitation, groundwaterinflow, and surface ornear-surface inflow) maygreatly diminish. In this instance, the amount of water lost throughevapotranspiration may exceed the rate of all water inflow to thewetland. Water losses through evapotranspiration can result inextreme declines in the water table and a de-saturation of thewetland.The relative importance of water inflow and water outflow, along withthe topographic and geologic setting, determines the type andcharacteristics of the wetland that may form at a given location. Anumber of wetland classification systems have been developed thatgroup wetlands based on topographic position in the landscape, watersource, and hydrodynamics (Novitski, 1979; Brinson, 1993). Fourcommonly found wetland systems in Michigan are surface waterdepression wetlands, groundwater slope wetlands, groundwaterdepression wetland, and surface water slope wetlands (Figures 2.2through 2.5). Wetlands that receive water primarily from precipitationhave been classified as surface water depressional wetlands.Wetlands for which groundwater is the predominant source of waterare classified as groundwater slope or groundwater depressionalwetlands. Wetlands which are dependent upon surface water infloware classified as either riverine or fringe wetlands along existingbodies of open water.Figure 2.2 shows a wetland that has formed in a topographicdepression. The primary sources of water are precipitation andsurface water runoff from the catchment area surrounding thewetland. Since the water level elevation in the wetland is greater thanthe elevation of the water table, water in the wetland moves towardthe water table, and groundwater is not a source of water for theFigure 2.1 - Wetland Hydrologic CycleSource: Carter, 1996, U.S. Geological Survey Water Supply Paper 2425.Chapter 2: Wetland Hydrology 11wetland. The outflow of water from this category of wetlands isevaporation from the water surface, transpiration from plants, andmovement of water to the underlying or adjoining aquifer. The soils orgeologic sediments which underlie the wetland may be predominantlyclay. The relatively low hydraulic conductivity of the sedimentrestricts, but does not prohibit, the movement of water from thewetland to the underlying aquifer. This category of wetlands isreferred to as surface water inflow or depressional wetlands. Thiscategory of wetlands may be found at any elevation, even inotherwise predominantly upland areas. These wetlands are moredependent on precipitation than other types of wetlands.Hillslopes between upland and lowland areas are another topographicsetting in which wetlands may form. Wetlands forming in these areasare referred to as groundwater slope wetlands. An example of thistype of wetland is shown in Figure 2.3. Groundwater whichdischarges along the hillslope as a seepor spring is the primary source of water tothis wetland. Overland flow andprecipitation may also contribute water tothese wetlands. In this setting, sedimentswhich have relatively low hydraulicconductivity such as clay or silt mayunderlie more permeable saturatedsediments, forming a perched aquifer.Groundwater would flow laterally, alongthe clay or silt layer, toward the hillslope,where it discharges as a seep or spring.This is referred to as a groundwaterseepage face. Groundwater slopewetlands may also occur where there arechanges in the hillside slope and may nothave perched groundwater conditions.Groundwater slope wetlands tend tohave relatively constant inflow of water ifthe aquifer responsible for the watersource is readily recharged orgroundwater moves through the aquiferat a relatively high rate. In this case, thewetland would be relatively unaffected byseasonal demands byevapotranspiration. If a shallow perchedaquifer provides water to the seep, thewetland soils may become dry duringportions of the growing seasonbecause of evapotranspiration in theseepage area. Adapted from Novitski, 1979.Adapted from Novitski, 1979.Figure 2.2 Surface water depression wetlandFigure 2.3 - Groundwater slope wetland12 MDEQ WETLAND IDENTIFICATION MANUALGroundwater slope wetlands generally have a surface water outlet.The size of these wetlands depends on the quantity of groundwaterdischarge and the slope of land surface downgradient of the seepageface or spring.Figure 2.4 shows a wetland formed in a topographic depression whichmay be in a lowland area. For this category of wetlands, the primarysources of water are groundwater discharge to the wetland,precipitation, and surface water runoff from the catchment areasurrounding the wetland. Since the water table elevation is higherthan the water level elevation in the wetland, groundwater movesfrom the adjoining and underlying aquifer toward the wetland. Theoutflow of water is from evaporation from the water surface andtranspiration from plants. These wetlands may not have any surfacewater outlets. This category of wetlands is referred to as groundwaterdepression wetlands. While they can exist at any elevation, thesewetlands are typically found in relativelylow-lying areas.Another category of wetlands is referredto as surface water slope wetlands(example shown in Figure 2.5). Surfacewater slope wetlands receive waterprimarily from the flooding of lakes orrivers, and the water can readily drainback into lakes or rivers as the surfacewater stages decline. Within floodplains,the flooding occurs infrequently.However, lakeside wetlands may beflooded permanently. These areas nearsurface water bodies are generally areasof regional or local groundwaterdischarge. The discharging groundwateris an important, consistent source ofwater to these wetlands.Riverine wetlands form as linear strips,generally paralleling river and streamchannels. These wetlands are found atlower elevations in a floodplain and tendto be more frequently inundated and fora longer duration than areas at slightlyhigher elevations.Fringe wetlands occur adjacent to lakeswhere water moves in and out of thewetland from the effects of wind, waves,and seiches. This is especially true forAdapted from Novitski, 1979.Adapted from Novitski, 1979.Figure 2.4 - Groundwater depression wetlandFigure 2.5 - Surface water slope wetlandChapter 2: Wetland Hydrology 13wetlands that have formed near the Great Lakes. Lakes that are toosmall to develop frequent seiches would not support fringe wetlands;such lakeside wetlands would fall into either the surface water orgroundwater inflow category described above.It is possible for the source of water to wetlands to change during wetand dry climatic cycles. As an example, the wetlands shown inFigures 2.2 and 2.4 might depict the same wetland, but underdifferent climatic conditions. Figure 2.2, with the low water table andinflow by surface water and precipitation only, may represent relativelydry or drought conditions. The conditions shown in Figure 2.4, withthe high water table and surface water, precipitation, and groundwaterinflow, may represent the same wetland during wet climatic periods.B. Field Indicators of Wetland HydrologyThe following field indicators of wetland hydrology are from theUSACE manual. The indicators can be quickly assessed and providesupport that inundation or soil saturation has occurred at a site.Although some indicators are not necessarily indicative of hydrologicevents that only occur during the growing season, they do provideevidence that inundation and/or soil saturation has occurred at a site.The use of these field indicators requires on-site observations.1. Primary indicators of wetland hydrology:a. Visual observation of inundation - The most obvious and revealinghydrologic indicator may be simply observing the areal extent ofinundation. However, because seasonal conditions and recentweather conditions can contribute to surface water being present on anon-wetland site, both should be considered when applying thisindicator.b. Visual observation of soil saturation - Examination of this indicatorrequires digging a soil pit to a depth of 16 inches and observing thelevel at which water stands in the hole after sufficient time has beenallowed for water to drain into the hole. The required time will varydepending on soil texture. In some cases, the upper level at whichwater is flowing into the pit can be observed by examining the wall ofthe hole. This level represents the depth to the water table. Thedepth to saturated soils will always be nearer the surface due to thecapillary fringe.For soil saturation to impact vegetation, it must occur within a majorportion of the root zone (usually within 12 inches of the surface) ofthe prevalent vegetation. The major portion of the root zone is thatportion of the soil profile in which more than one half of the plant roots14 MDEQ WETLAND IDENTIFICATION MANUALoccur. CAUTION: In some heavy clay soils, water may not rapidlyaccumulate in the hole even when the soil is saturated. If water isobserved at the bottom of the hole but has not filled to the 12-inchdepth, examine the sides of the hole and determine the shallowestdepth at which water is entering the hole. When applying thisindicator, both the season of the year and preceding weatherconditions must be considered.c. Watermarks - Watermarks are most common on woodyvegetation. They occur as stains on bark or other fixed objects (e.g.,bridge pillars, buildings, fences, etc.). When several watermarks arepresent, the highest reflects the maximum extent of recent inundation.d. Drift lines - This indicator is most likely to be found adjacent tostreams or other sources of water flow in wetlands. Evidenceconsists of deposition of debris in a line on the surface or as debrisentangled in above ground vegetation or other fixed objects. Debrisusually consists of remnants of vegetation (branches, stems, andleaves), sediment, litter, and other waterborne materials depositedparallel to the direction of water flow. Drift lines provide an indicationof the minimum portion of the area inundated during a flooding event;the maximum level of inundation is generally at a higher elevationthan that indicated by a drift line.e. Sediment deposits - Plants and other vertical objects often havethin layers, coatings, or depositions of mineral or organic matter onthem after inundation. This evidence may remain for a considerableperiod before it is removed by precipitation or subsequent inundation.Sediment deposition on vegetation and other objects provides anindication of the minimum inundation level. When sediments areprimarily organic (e.g., fine organic material, algae), the detritus maybecome encrusted on or slightly above the soil surface afterdewatering occurs.f. Drainage patterns within wetlands - This indicator, which occursprimarily in wetlands adjacent to streams, consists of surfaceevidence of drainage flow into or through an area. In some wetlands,this evidence may exist as a drainage pattern eroded into the soil,vegetative matter (debris) piled against the thick vegetation or woodystems oriented perpendicular to the direction of water flow, or theabsence of leaf litter. Scouring is often evident around roots ofpersistent vegetation. Debris may be deposited in or along thedrainage pattern.CAUTION: Drainage patterns also occur in upland areas afterperiods of considerable precipitation; therefore, topographic positionmust also be considered when applying this indicator. Primary Indicators of WetlandHydrololgy:a. Visual observation of inundationb. Visual observation of soil saturationc. Watermarksd. Drift markse. Sediment depositsf. Drainage patterns within wetlandsChapter 2: Wetland Hydrology 152. Supplemental indicators of wetland hydrology:In addition to the primary indicators of wetland hydrology, the MDEQmay also consider the following site conditions as supplementalindicators to support evidence of wetland hydrology:a. Oxidized rhizospheres (root channels) associated with living plantroots in the upper 12 inches of the soil Oxidized rhizospheressurrounding living roots are acceptable hydrology indicators on acase-by-case basis and may be useful in groundwater systems. Usecaution that rhizospheres are not relicts of past hydrology.Rhizospheres should also be reasonably abundant and within theupper 12 inches of the soil profile. Oxidized rhizospheres must besupported by other indicators of hydrology, such as the FAC-neutraltest if hydrology evidence is weak.b. Water-stained leaves - The presence of stained vegetation can beused as a secondary indicator of wetland hydrology. The physicalappearance of leaves resulting from the continued presence of waterand anaerobic processes will often darken leaf surfaces.Comparisons can be made to leaves occurring within obvious areasof upland vegetation.c. Local soil survey hydrology data for identified soils - Ingroundwater-driven systems, which lack surface indicators of wetlandhydrology, it is acceptable to use local NRCS soil survey informationto evaluate the hydrology parameter in conjunction with otherinformation, such as the FAC-neutral test. Use caution in areas thatmay have been recently drained.d. FAC-neutral test The FAC-neutral test results in a positivesecondary indicator of hydrology when more of the dominant plantspecies have a wetland indicator category that is wetter than FAC.(For an explanation of wetland indicator categories for plants, seeChapter 3). The FAC-neutral test considers FAC species (FAC-, FAC,or FAC+) as neutral and does not utilize them. Rather, theabundance of OBL, FACW+, FACW, and FACW- species areweighed against the abundance of UPL, FACU-, FACU, and FACU+species (OBL + FACW species > FACU + UPL species) to determinewhether the vegetation meets the FAC-neutral test.e. Bare soil areas - Wetlands that contain standing water for arelatively long duration may have areas of bare or essentially baresoil. Bare soil areas can be a result of surface flows carrying awayground litter or the presence of standing water within localdepressions for a relatively long time with limited inputs of plant littermaterial.16 MDEQ WETLAND IDENTIFICATION MANUALf. Morphological plant adaptations Some plant species haverecognizable physical characteristics, such as butressed trunks, thatreflect their ability to occur and survive in wetland conditions. Severaltypes of morphological adaptations are listed and described inAppendix C.Supplemental Indicators of WetlandHydrololgy:a. Oxidized rhizopheres (root channels) associated with living plant roots in the upper 12 inches of the soilb. Water-stained leavesc. Local soil survey hydrology data for identified soilsd. FAC-neutral teste. Bare soil areasf. Morphological plant adaptations