CADDIS Volume 2: Sources, Stressors & Responses
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Author: C.R. Ziegler
Movement of water through stream and river channels influences all processes and biota within. The terms “flow,” “discharge,” and “streamflow” often are used interchangeably to represent water volume passing a fixed channel location per unit time. In 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). Velocity can be thought of as a two-dimensional variable (length over time), whereas discharge is a four-dimensional variable (volume over time). The significance of this distinction becomes evident when considering multiple sites along a stream reach: in the riffle, velocity is high, and in the pool immediately upstream, velocity is low, but for both channel cross sections, discharge remains the same (assuming no inputs or diversions between the riffle and pool).
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 regional and temporal variability of flow 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. For example, human activities may change the amount of water reaching a stream, divert flow through manmade channels (Figure 1), or 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 to 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.
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 (see below).
This module provides advice for deciding whether to include flow alteration in your list of candidate causes, as well information on ways to measure flow alteration. 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 diagram for flow alteration (Figure 2). You may go directly to a specific section of interest by clicking on the tabs above.
The checklist below will help you identify key data and information useful for determining whether to include flow alteration among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate flow alteration as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.
Consider listing flow alteration as a candidate cause when the following sources and activities, site evidence, and biological effects are present:
- Point source inputs
- Water withdrawals
- Land cover alteration (e.g., impervious surfaces)
- Storm drain systems
- Agricultural tile drainage
- Channel alteration
- Channel erosion
- Scouring and incision
- Dry stream
- Channel features incongruous with observed flow
- Discharge data inconsistencies
- Reduced productivity
- Changes in community composition
- Increased generalists and decreased specialists
- Replacement of native species by invasive or exotic species
- Disrupted reproductive cycles
- Decreased taxonomic richness and diversity
Consider contributing, modifying, and related factors as candidate causes when flow alteration is selected as a candidate cause:
- 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 3).
- Ionic strength: Increased discharge can dilute toxic substances (e.g., ions and metals) and decrease toxicity, whereas decreased discharge can have the opposite effect. However, increased discharge often results from increased surface runoff, and this runoff can deliver increased loads of nutrients, sediment, ions, metals, and other toxics to streams. In addition, decreased flow velocity may allow increased deposition of particle-associated toxic substances.
- pH: See ionic strength (above).
- Metals: See ionic strength (above).
- Nutrients: See ionic strength (above).
- Sediments: See ionic strength (above) and physical habitat (below).
- Physical 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 (Figure 4). 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.
- Unspecified toxic chemicals: See ionic strength (above).