Causal Analysis/Diagnosis Decision Information System (CADDIS)

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CC.7. Flow Alteration

Movement of water through stream and river channels influences all processes and biota within. The terms “flow,” “discharge,” and “streamflow” are often used interchangeably and represent water volume passing a fixed channel location per unit time. For the purposes of this module, we broadly define “flow” to include current velocity, volume as a function of time, large event frequency, and other parameters that might be said to characterize a waterbody’s flow regime. We use the term “discharge” specifically in reference to volume as a function of time, reported commonly as cubic meters per second (m³/s), cubic feet per second (cfs), gallons per minute (gpm), or acre-feet per year (ac-ft/yr).

Flow characteristics vary throughout a watershed, longitudinally along a stream's channel, and laterally from channel to floodplain, as a function of landscape features. The variability of flow, both regionally and temporally, results from variance in rainfall patterns, vegetation, development, geology and other watershed characteristics. Biological characteristics at a given site relate to volume, velocity, and variance of flow (including event frequency, duration, timing, and rates of change). It may be appropriate to consider these components of flow in combination or individually as separate candidate causes.

Flow alteration refers to modification of flow characteristics, relative to reference or natural conditions. Human activities can significantly alter flow, and may lead to biological impairment. Human activities, for example, may change the amount of water reaching a stream, divert flow through manmade channels (Image CC.7-1), and alter the shape and location of streams. Changes to stream and river flow characteristics may benefit some aquatic organisms while harming others, thereby changing biotic community composition. Stress related to flow alteration is closely tied with temporal variability and regional flow characteristics. For example, arid and seasonal streams of the western U.S. behave differently than temperate streams of the mid-Atlantic, and significant stressors in one region may be trivial for the biological community of the other.

Advice for deciding whether to include flow alteration, or specific attributes of a given flow regime, in your list of candidate causes is provided in Section CC.7.1. Ways to measure flow alteration are discussed in Section CC.7.2. You may go directly to a specific section of interest by clicking on the topic outline in the right navigation bar.

CC.7.1 What to Consider When Determining if Flow Alteration Should Be Included as a Candidate Cause

Flow alteration is addressed in this module as a proximate stressor that should be listed as a candidate cause when potential human sources and activities, site evidence, and biological effects support portions of source-to-impairment pathways as illustrated in the conceptual model diagram for flow alteration (Figure CC.7-1).

Temporal and spatial variability make it difficult to characterize a watershed's natural flow regime. Simple rules of thumb, such as minimum required discharge for a given stream or river, may not be sufficient when linking flow regime to ecological function (Arthington et al., 2006). Aggregate flow characteristics such as discharge measured at a gauge depend on watershed-wide landscape features and precipitation patterns in the entire watershed. However, localized flow information, such as water velocity and depth—that is, reach-scale or hydraulic properties—may reflect the condition of a particular reach as well as watershed properties.

Along with challenges described above, flow alteration often interacts with other stressors to cause impairment. Flow is connected to multiple biotic and abiotic components of aquatic ecosystems (Power et al., 1995), and causal assessors should consider potential interactions when listing candidate causes; for example:

Dissolved oxygen — Oxygen enters streams in various ways including atmospheric diffusion and entrainment from riffles and waves. Changes to flow may reduce surface area and turbulence, which can decrease dissolved oxygen and stress organisms. In this case, dissolved oxygen would be considered the proximate stressor, while flow alteration is a step in the causal pathway. The two stressors also may act jointly. For example, when oxygen concentrations in the water column are low, less oxygen flows past the respiratory structures of aquatic organisms. In this case, both current velocity and dissolved oxygen concentration may determine the fate of organisms without actively ventilated gills (Figure CC.7-2).

Structural habitat — Flow is closely tied to fluvial geomorphological processes and related habitat features. Physical channel processes such as deposition and erosion partly define a channel’s dynamic and natural condition. Flow transports material (e.g., cobbles, sediment, woody debris) through stream and river systems, and flow reshapes channels and floodplains, for example, by encouraging channel meanders (Image CC.7-2). Conversely, structural habitat features such as boulders and woody debris may affect flow at local and reach levels by altering velocity and water depth. Structural habitat and flow alteration can serve as proximate stressors, while significantly influencing each other through physical processes as steps in a causal chain. To learn more about geomorphology and the interplay between flow and structural habitat, see Leopold et al., 1995 and FISRWG, 1998.

Temperature — Flow alteration may interact with water temperature in a variety of ways. For example, altered overland flow may decrease groundwater recharge and, consequently, lead to reduced cold groundwater inputs to streams, potentially increasing stream temperatures. For this particular example, flow alteration might be referred to as a step in the causal pathway leading to increased water temperature—the proximate stressor.

Toxic substances — Increased discharge can dilute toxic substances (e.g., ions and metals) and decrease toxicity, whereas decreased discharge can have the opposite effect. In addition, decreased flow velocity may allow increased deposition of particle-associated toxic substances.

CC.7.1.1 Checklist of Sources, Site Evidence, and Biological Effects

A checklist is provided below to help you identify key data and information useful for determining whether to include or exclude flow alteration among your candidate causes; the title for each column is linked to more detailed descriptions. The list is intended to guide you in collecting evidence to eliminate or enhance support for flow alteration as a candidate cause. You may be aware of other situations in which flow alteration should be eliminated or included as a candidate cause; please send us your insights using the comment section.

Consider listing flow alteration as a candidate cause based on the presence of sources and activities, site evidence, and biological effects:

Consider contributing, modifying, and related factors as candidate causes when flow alteration is selected as a candidate cause:

• Dissolved oxygen
• Ionic strength
• pH
• Metals
• Nutrients
• Sediments
• Structural habitat
• Temperature
• Unspecified toxic chemicals

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CC.7.1.2 Sources and Activities that Suggest Listing Flow Alteration as a Candidate Cause

The natural flow regime of a stream or river may be altered by various human activities, within the channel or the watershed. The more extensive the relevant sources and activities, the more likely increased flow alteration will impair surface waters.

Point source inputs — Anthropogenic inputs to streams include untreated sewage and stormwaters (Image CC.7-3), wastewater treatment plants and industrial operations; these inputs can increase flow or provide a source of flow in an otherwise dry channel. Redirecting flow from one watershed to another, or transbasin diversions, also may increase flow in one stream (point source), while decreasing flow in another (water withdrawal; see below).

Water withdrawal — Surface water and groundwater withdrawals can alter flow by reducing water volumes in streams. Withdrawals may return to the surface/groundwater system at a point further downstream, be removed from the watershed through transpiration by crops, lawns or pastures, or be transferred to another watershed altogether (e.g., water transferred to a different watershed for drinking supply).

Land cover alteration — Changes in land cover (Images CC.7-4) alter hydrologic processes including infiltration, uptake of runoff by vegetation, and the efficiency of overland flow. Surfaces with low permeability decrease watershed-wide infiltration and increase the efficiency or speed at which surface runoff reaches streams. Impervious surfaces generally offer less resistance to overland flow than do areas covered by natural vegetation. Thus, flow is flashier with higher peak flows, shorter duration flow events, and more frequent high flows. Additionally, decreased watershed infiltration often decreases groundwater recharge, consequently decreasing stream baseflow. Impervious surfaces, and other reduced permeability surfaces, include roads, parking lots, roofs, and compacted surfaces such as pastures and logging access roads.

Storm drain systems — Stormwater systems (Image CC.7-5) and roadside ditches and culverts (Image CC.7-6) frequently accompany land cover alterations and urbanization. Storm drain systems generally reduce groundwater recharge and increase the efficiency with which precipitation reaches streams by reducing overland flow resistance, in contrast to natural surfaces (e.g., forested areas and natural channels), and by providing direct travel routes for runoff to streams via conduits with smooth surfaces (e.g., concrete and metal). The resulting reduction in groundwater recharge and related subsurface flows may decrease stream baseflows and increase the likelihood of flow intermittency or cessation.

Agricultural tile drainage — Agricultural drainage systems are often used to intentionally reduce soil moisture for optimal growing conditions by moving precipitation or irrigation waters from subsurface soils, through pipes, and eventually into ditches or streams thereby increasing flow. Like storm drain systems, tile drains reduce groundwater recharge.

Channel alteration — Structural habitat changes include straightening or restructuring natural watercourses. This can involve adding rip-rap to stabilize a stream bank, installing a dam or road crossing, or removing large woody debris. Reduced sinuosity may shorten the distance water travels and increase water velocity. Engineered channels designed to prevent overbank flooding eliminate or reduce floodplain connectivity (some organisms, such as certain species of spawning fish, rely on floodplain habitats, which are sometimes only made available when storm flows rise above channel banks). Channelization also alters channel bathymetry and gradient, thereby reducing structural habitat heterogeneity and riffle/pool variability.

Impoundment — Detention basins, retention basins, and dams (Images CC.7-7 and CC.7-8) can affect flow regime in various ways, via actively controlled or passive-release mechanisms. For example, detention basins tend to reduce peak flows during precipitation events or prevent water from reaching a stream during periods of low flow. Similarly, storage dams may reduce or prevent seasonal variation of downstream flow, and decrease the frequency of floodplain inundation. However, hydroelectric dams may increase variance in flow if they are used for peaking power.

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CC.7.1.3. Site Evidence that Suggests Listing Flow Alteration as a Candidate Cause

In addition to observations of sources discussed above, the following observations may suggest listing flow alteration as a candidate cause. Note, however, that single point-in-time observations also may be indicative of natural channel processes. Flow characteristics, structural habitat conditions, and associated biological attributes at a given point in a stream depend on various in-stream and watershed-wide factors upstream, which can change with time. Therefore, the following site observations should be considered in the context of the associated ecosystem and other local factors, such as precipitation and geology.

Channel erosion — Bank erosion and instability, undercut banks, and exposed roots, particularly in channel areas not confined to outside bends, may suggest that peak flows have increased in magnitude (discharge and velocity) and/or frequency (Image CC.7-9).

Scouring and incision — Channel scouring, incision, or downcutting suggest altered flow conditions characterized by increased peak flow magnitude (discharge and velocity) and/or frequency of high flow events. Scouring flows may act to dislodge organisms (i.e., flow as a proximate cause of impairment) and to alter substrate composition or structural habitat (i.e., flow as a step in a causal chain).

Dry stream — A dry stream bed may suggest including flow alteration as a candidate cause. Land cover changes, for example, can eliminate between-storm baseflow. Note, however, that some streams, particularly in the western U.S., are naturally intermittent due to seasonal and regional precipitation patterns. Whether the dry channel is natural or un-natural, flow alteration may be included because aquatic organisms often are sensitive to the timing and duration of zero flow conditions.

Channel features incongruous with observed flow — Observation of a “normal” precipitation event—for example, an event of magnitude that might happen about once per year or once every other year—for which flow levels do not reach bank-full channel features and/or floodplain terraces, may suggest that larger events (altered flow conditions) are dictating channel geometry and evolution.

Discharge data inconsistencies — A plot showing both precipitation and stream discharge (see Section CC.7.2. Ways to Measure Flow Alteration) may alert causal assessors of certain flow alterations. Zero baseflow between precipitation events may indicate that land cover alterations (e.g., impervious surfaces) have created a flashy hydrologic system, marked by higher peak flows, but reduced baseflow. Conversely, if discharge is relatively constant regardless of precipitation, wastewater dominance or control of flow by a dam might be altering flow characteristics.

Anecdotal information may strengthen site observation evidence. A classic example is that of the scientist walking up to a dry stream and a local resident curiously joining the scientist streamside; the resident says, "I remember 50 years ago when this creek had water flowing through it everyday of the year, and now it's bone dry unless it rains." Such information would be sufficient to include flow as a candidate cause, but the credibility of anecdotal information must be checked during the causal analysis.

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CC.7.1.4. Biological Effects that Suggest Listing Flow Alteration as a Candidate Cause

Fish, invertebrates, and aquatic plants vary in their preferences for specific flow characteristics. Here we present examples of biological effects of flow alteration, reported in several synthesis documents linking the disciplines of hydrology and ecology (The Nature Conservancy, 2006; Biggs et al., 2005; Bragg et al., 2005; Bunn and Arthington, 2002; Poff et al., 1997; Poff and Ward, 1989). The general trend among scientists is to connect flow alteration parameters with biological effects. Effects typically are described in the context of species traits, functional adaptations, life history characteristics, or community structure (Roy et al., 2005; Lytle and Poff, 2004; Goldstein and Meador, 2004). The following examples are categorized in terms of common flow alteration parameters.

Changes in magnitude and duration of low flows — Cessation of flow, extreme low flows, or prolonged duration of low flow conditions can reduce overall habitat availability by decreasing water volume and wetted channel area. This alteration has been linked to reduced total stream productivity, elimination of large fish, changes in taxonomic composition of fish communities, fewer species of migratory fish, fewer fish per unit area, and a greater concentration of some aquatic organisms (potentially benefiting predators). Prolonged duration of low flows tends to favor invertebrate and fish species that prefer standing-water habitats or species classified as generalists. Conversely, extreme low flows that are not unusual for the channel of interest may benefit aquatic systems, by purging invasive or non-native species maladapted to such conditions.

Changes in frequency and magnitude of peak flows — High-flow events can physically remove species from the channel to a downstream location. The literature generally refers to this process as dislodgement, wash-out, scouring, or flushing of organisms. Mobilization of pebbles, sediment, woody debris, and plant material, in addition to movement of water itself, also can dislodge organisms. More frequent high-flow events can decrease species richness by eliminating or reducing populations that do not fare well under high-flow conditions. Invertebrate assemblages consisting of species with long life cycles may shift compositionally to include more species with relatively short life cycles. Conversely, fewer high-flow events and peak flow events that are lower in magnitude may disconnect the channel from its floodplain, reducing access to fish spawning habitat and juvenile fish nursery areas.

Altered seasonality of flows — Many aquatic organisms rely on consistent seasonal flow patterns (e.g., flow increases with spring snow melt) to cue life history events. Altered or reduced seasonality of flows, including changes in the timing of rising flows and flow peaks, may disrupt natural cues for invertebrate life cycles and for migration, spawning, and egg hatching of fish. For example, Peckarsky et al. (2000) discuss the life cycle traits and cues common to the mayfly genus Baetis in connection to flow regime. Specifically, baetid mayflies oviposit on rocks based partly on the timing of rock appearance above the water line, and therefore, seasonal changes in water depth play a role in cueing reproductive processes of baetid mayflies.

Changes in flow variability — The natural variability of a stream's flow regime usually includes peak flow diversity combined with occasional periods of drought. Dams with regulated releases can stabilize water flow, thus reducing variability. This flow stabilization may provide an opportunity for invasive or exotic species to establish and displace native species. Additionally, native specialist organisms, adapted to a particular combination of high and low flows, may be replaced by generalist species, which may not otherwise compete successfully with native species under more natural flow conditions. Decreased variability in flow can reduce fish populations and diversity of the invertebrate community.

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CC.7.1.5. Site Evidence that Supports Excluding Flow Alteration as a Candidate Cause

There are no site observations that specifically provide evidence of the absence of flow alteration. General reasons for excluding a candidate from the list are described in Step 2.2 of the Step-by-Step guide and in Tips for Listing Candidate Causes.

We strongly caution against using benchmarks of effects (e.g., water quality criteria) as evidence for excluding flow alteration from your initial list of candidate causes, because different species have different flow requirements and different sites have different naturally occurring levels of flow.

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CC.7.2. Ways to Measure Flow Alteration

In the context of causal assessment, water depth, volume, velocity, and discharge, each of which varies with time and space, are considered part of the flow regime. Discharge is often a primary focus of hydrologic studies (especially large scale studies), but the existence, quality, and accessibility of hydrologic data may dictate which measures of flow alteration are used in any given causal assessment.

Discharge

Continuous discharge rarely is measured directly. Instead, flow gauges typically measure instantaneous water depth (i.e., stage) at intervals (e.g., every 5 to 60 minutes); a rating curve (plot of discharge versus stage height) then can be used to convert stage to discharge. To construct a rating curve, measurements of discharge are made for a channel cross-section under a range of flow conditions (e.g., baseflow, small storms, and large storms). Discharge can be estimated by measuring flow velocity and water depth at several points across the width of a channel [for more information on field hydraulic measures, see Rantz (1982)]. The U.S. Geological Survey (USGS) is the primary U.S. agency that collects flow data. In addition, local government agencies, state biologists, consulting firms, or other entities may provide causal assessors with discharge data over time or rating curves and measured depth of flow; causal assessors should consider differences between raw and calculated data in terms of accuracy, collection techniques, and underlying calculation assumptions.

Hydrographs, plotted as discharge versus time (Figure CC.7-3), can be used to characterize flow at a given stream cross-section. The shape of a hydrograph following a precipitation event reveals information about the contributing watershed's flow regime, and is characterized by several parameters, including: volume of flow, or the area under the hydrograph; magnitude, or peak flow; duration of the event above a certain flow; rate of change from, say, low to peak discharge and back again (i.e., a flashy system); and lag time, or time between the rainfall center of mass and flow volume center of mass.

For causal assessment, hydrographs of reference versus impaired streams or hydrographs before and after a disturbance might be compared to determine, for example, the impacts of urbanization (Figure CC.7-4), dams, or channelization.

Figures CC.7-3 and CC.7-4 show discharges resulting from single rain events. Sometimes, it is advantageous to analyze a hydrograph for one or more water years (typically, October 1st of year X, to September 30th of year X+1) to better understand frequency of small events, seasonal variation of flow, and baseflow levels or groundwater inputs between precipitation events.

Statistical Measures

Long-term flow data may be used to develop statistical descriptors of flow regime. A common statistical measure of flow is event frequency (sometimes expressed as recurrence interval or return period). A 10-year flow event is an event with a magnitude predicted to recur once every ten years; such an event has a 10% chance of happening in any given year. USGS provides guidance for estimating flow frequency (U.S. Interagency Advisory Committee on Water Data, 1982). Flood management often focuses on large events with low frequency (e.g., 10- to 100-year events), although smaller events with greater frequency play an equally important role in geomorphological and ecological processes. Cunnane (1978) provides an equation for estimating sub-annual return periods - that is, recurrence of small events within a year.

Duration was mentioned above in the context of a single event (i.e., length of time flow is above a certain magnitude). Flow duration also can be calculated for longer time periods. For example, flow duration can be defined as the length of time (generally, the number of days) per year in which a stream's discharge is greater than a particular value.

Software has been developed to calculate hydrologic statistics. PEAKFQ is an application based on USGS's guidelines for determining flood flow frequency (U.S. Interagency Advisory Committee on Water Data, 1982), and is publicly available at U.S. Geological Survey-PEAKFQ. USGS recommends having ten years of flow data before conducting a basic determination of event frequency. Such data might be in the form of ten consecutive annual peak flow values - that is, the highest estimated discharge for each of ten years, input as a list into the application. The Nature Conservancy also has developed software for estimating statistical indicators describing flow, and the Indicators of Hydrologic Alteration (IHA) software application and supporting information can be downloaded from The Nature Conservancy. IHA requires daily discharge data for input (refer to the IHA user's manual for additional information including data requirements; The Nature Conservancy, 2006).

Hydrologic Models

The location of flow gauges along a stream or river may not coincide with biological sampling sites. Furthermore, data from an impaired site may not be matched with comparable reference site data. If a flow gauge is downstream of a biological sampling site, it may be appropriate to scale down flow estimates for the biological sampling site, using a ratio of watershed area between the two locations. Alternatively, a computer model characterizing a watershed's hydrologic behavior may assist causal assessment by providing data to compare with observed data, and allowing further understanding of flow regimes. Model inputs might include precipitation, watershed shape and size, infiltration rates, and lengths and roughness of flow paths. These data also may allow for simple analyses. For example, an investigator could compare the percent of total precipitation reaching stream channels among watersheds with varying amounts of impervious surface area. Hydrologic data also may allow investigators to develop more complicated simulations that depict a range of flow characteristics. Hydrologic computer models can be used to compare pre- and post-development flow regimes, to compare reference versus impaired watershed flow characteristics, and to consider potential impacts of restoration designs. Modeling software continues to evolve as computational technology improves. For example, the U.S. EPA and the U.S Army Corps of Engineers Hydrologic Engineering Center (HEC) provide modeling applications and supporting materials (BASINS and HEC, respectively).

Conducting a hydrologic study often involves developing a map of the watershed, as hydrologic modeling frequently entails using Geographic Information Systems (GIS) to characterize watershed features. Examples of GIS components in hydrologic modeling include digital elevation models (DEMs or topography) and soil-type layers, which are sometimes made available by local government agencies. Causal assessors may search for existing hydrologic studies and models of impaired watersheds, perhaps conducted or contracted by local government agencies.

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Arthington, AH; Bunn, SE; Poff, NL; et al. (2006) The challenge of providing environmental flow rules to sustain river ecosystems. Ecol Appl 16 (4): 1311-1318.

Biggs, BJF; Nikora, VI; Snelder,TH. (2005) Linking scales of flow variability to lotic ecosystem structure and function. Riv Res App 21 (2-3): 283-298.

Bragg, OM; Black, AR; Duck, RW; et al. (2005) Approaching the physical-biological interface in rivers: a review of methods for ecological evaluation of flow regimes. Prog Phys Geog 29 (4): 506-531.

Bunn, SE; Arthington, AH. (2002) Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity. Environmental Management 30 (4): 492-507.

Cunnane, C. (1978) Unbiased plotting positions: a review. J Hydro 37: 205-222.

FISRWG (Federal Interagency Stream Restoration Working Group) (1998) Stream corridor restoration: principles, processes, and practices. GPO Item No.0120-A; SuDocs No. A 57.6/2:EN 3/PT.653.

Goldstein, RM; Meador, MR. (2004) Comparisons of fish species traits from small streams to large rivers. Trans Amer Fish Soc 133 (4): 971-983.

Jaag, O; Ambühl, H. (1964) The effect of the current on the composition of biocoenoses in flowing water streams. Adv Water Pollut Res 1:39-49.

Leopold, LB; Wolman, MG; Miller, JP. (1995) Fluvial processes in geomorphology. New York, NY: Dover Publications.

Lytle, DA; Poff, NL. (2004) Adaptation to natural flow regimes. Trends in Ecology & Evolution 19 (2): 94-100.

Peckarsky, BL; Taylor, BW; Caudill, CC. (2000) Hydrologic and behavioral constraints on oviposition of stream insects: implications for adult dispersal. Oecologia 125:186-200.

Poff, NL; Allan, JD; Bain, MB; et al. (1997) The natural flow regime. Bioscience 47 (11): 769-784.

Poff, NL; Ward, JV. (1989) Implications of streamflow variability and predictability for lotic community structure: a regional-analysis of streamflow patterns. Can J Fish Aquat Sci 46 (10): 1805-1818.

Power, ME; Sun, A; Parker, G; et al. (1995) Hydraulic food-chain models. Bioscience 45 (3): 159-167.

Rantz, SE. (1982) Measurement and computation of streamflow: volume 1 measurement of stage and discharge; volume 2: computation of discharge. U.S. Geological Survey, Water Supply Paper 2175.

Roy, AH; Freeman, MC; Freeman, BJ; et al. (2005) Investigating hydrologic alteration as a mechanism of fish assemblage shifts in urbanizing streams. J North Amer Benthol Soc 24(3): 656-678.

The Nature Conservancy. (2006) Indicators of hydrologic alteration: version 7 user's manual.

US Interagency Advisory Committee on Water Data. (1982) Guidelines for determining flood flow frequency: bulletin 17B of the hydrology subcommittee. Office of Water Data Coordination, U.S. Geological Survey, Reston, VA.

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