hydrological processes return to normal levels

Increased generalists and decreased specialists.Channel features incongruous with observed flow.Land cover alteration (e.g., impervious surfaces).For more information on specific entries, go to the When to List tab.Ĭonsider listing flow alteration as a candidate cause when the following sources and activities, site evidence and biological effects are present: Sources and Activities The list is intended to guide you in collecting evidence to support, weaken or eliminate flow alteration as a candidate cause. The checklist below will help you identify key data and information useful for determining whether to include flow alteration among your candidate causes. Flow alteration 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 (see Figure 2). This module provides advice for deciding whether to include flow alteration in your list of candidate causes, as well information on measuring flow alteration. Checklist of Sources, Site Evidence and Biological Effects Causal assessors should consider potential interactions when listing candidate causes (see below). Flow is connected to multiple biotic and abiotic components of aquatic ecosystems (Power et al. However, localized flow information (e.g., water velocity and depth) may reflect the condition of a particular reach as well as watershed properties.Īlong with challenges described above, flow alteration often interacts with other stressors to cause impairment. Aggregate characteristics (e.g., discharge measured at a gauge) depend on watershed-wide landscape features and precipitation patterns. Rules of thumb (e.g., minimum required discharge) may be insufficient when linking flow regime to ecological function (Arthington et al. Temporal and spatial variability make it difficult to characterize a watershed's flow regime. Significant stressors in one region may be trivial for the biological community of the other. behave differently than temperate streams of the mid-Atlantic. For example, arid and seasonal streams of the western U.S. Stress related to flow alteration is closely tied to temporal variability and regional flow characteristics. Changes to streamflow characteristics may benefit some aquatic organisms while harming others, thereby changing biotic community composition. For example, human activities may change how much water reaches a stream, divert flow through manmade channels (Figure 1) or alter stream shape and location. Human activities can significantly alter flow and may lead to biological impairment. It may be appropriate to consider these different flow parameters as separate candidate causes.įlow alteration refers to modification of flow characteristics, relative to reference or natural conditions. Biological characteristics at a given site relate to flow volume, velocity and variance (including rates of change, frequency, duration and timing). Regional and temporal flow variability results from variance in rainfall patterns, vegetation, development, geology and other watershed characteristics. For both channel cross sections, however, discharge remains the same (assuming no inputs or diversions between the riffle and pool).įlow characteristics vary longitudinally along a stream's channel and laterally from channel to floodplain, as a function of landscape features. In the riffle, velocity is high, and in the pool immediately upstream, velocity is low. This distinction's significance becomes evident when considering multiple sites along a stream reach. Velocity can be thought of as a two-dimensional variable (length over time), whereas discharge is a four-dimensional variable (volume over time). Discharge is reported commonly as cubic meters per second (m 3/s), cubic feet per second (cfs), gallons per minute (gpm), or acre-feet per year (ac-ft/yr). We use the term “discharge” specifically in reference to volume as a function of time. We broadly define “flow” to include current velocity, volume as a function of time, large event frequency, and other parameters that characterize a flow regime. The terms “flow,” “discharge” and “streamflow” often are used interchangeably to represent water volume passing a fixed channel location per unit time. Movement of water through stream and river channels influences all processes and biota within.

The Chesapeake and Ohio Canal National Historical Park, Washington D.C., an example of a channelized overland flow diversion.

Hydrological processes return to normal levels