H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).

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The Natural Flow Regime

The flow regime is regarded by many aquatic ecologists to be the key driver of river and floodplain wet- land ecosystems. We have focused this literature review around four key principles to highlight the important mech- anisms that link hydrology and aquatic biodiversity and to illustrate the consequent impacts of altered flow regimes: Firstly, flow is a major determinant of physical habitat in streams, which in turn is a major determinant of biotic com- position; Secondly, aquatic species have evolved life history strategies primarily in direct response to the natural flow regimes; Thirdly, maintenance of natural patterns of longitu- dinal and lateral connectivity is essential to the viability of populations of many riverine species; Finally, the invasion and success of exotic and introduced species in rivers is facilitated by the alteration of flow regimes. The impacts of flow change are manifest across broad taxonomic groups including riverine plants, invertebrates, and fish. Despite growing recognition of these relationships, ecologists still struggle to predict and quantify biotic responses to altered flow regimes. One obvious difficulty is the ability to distin- guish the direct effects of modified flow regimes from im- pacts associated with land-use change that often accom- panies water resource development. Currently, evidence about how rivers function in relation to flow regime and the flows that aquatic organisms need exists largely as a series of untested hypotheses. To overcome these problems, aquatic science needs to move quickly into a manipulative or experimental phase, preferably with the aims of restora- tion and measuring ecosystem response.

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Basic principles and ecological consequences of altered flow regimes for aquatic biodiversity.

Summary
1. In an effort to develop quantitative relationships between various kinds of flow alteration and ecological responses, we reviewed 165 papers published over the last four decades, with a focus on more recent papers. Our aim was to determine if general relationships could be drawn from disparate case studies in the literature that might inform environmental flows science and management.

2. For all 165 papers we characterised flow alteration in terms of magnitude, frequency, duration, timing and rate of change as reported by the individual studies. Ecological responses were characterised according to taxonomic identity (macroinvertebrates, fish, riparian vegetation) and type of response (abundance, diversity, demographic parameters). A ‘qualitative’ or narrative summary of the reported results strongly corroborated previous, less comprehensive, reviews by documenting strong and variable ecological responses to all types of flow alteration. Of the 165 papers, 152 (92%) reported decreased values for recorded ecological metrics in response to a variety of types of flow alteration, whereas 21 papers (13%) reported increased values.

3. Fifty-five papers had information suitable for quantitative analysis of ecological response to flow alteration. Seventy per cent of these papers reported on alteration in flow magnitude, yielding a total of 65 data points suitable for analysis. The quantitative analysis provided some insight into the relative sensitivities of different ecological groups to alteration in flow magnitudes, but robust statistical relationships were not supported. Macroinvertebrates showed mixed responses to changes in flow magnitude, with abundance and diversity both increasing and decreasing in response to elevated flows and to reduced flows. Fish abundance, diversity and demographic rates consistently declined in response to both elevated and reduced flow magnitude. Riparian vegetation metrics both increased and decreased in response to reduced peak flows, with increases reflecting mostly enhanced non-woody vegetative cover or encroachment into the stream channel.

4. Our analyses do not support the use of the existing global literature to develop general, transferable quantitative relationships between flow alteration and ecological response; however, they do support the inference that flow alteration is associated with ecological change and that the risk of ecological change increases with increasing magnitude of flow alteration.

5. New sampling programs and analyses that target sites across well-defined gradients of flow alteration are needed to quantify ecological response and develop robust and general flow alteration–ecological response relationships. Similarly, the collection of pre- and post-alteration data for new water development programs would significantly add to our basic understanding of ecological responses to flow alteration.

https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/deltaflow/docs/exhibits/swrcb/swrcb_poff2010_2.pdf

Ecological responses to altered flow regimes: a literature review to inform the science and management of environmental flows

> * SUMMARY 1. This paper introduces a new approach for setting streamflow-based river ecosystem management targets and this method is called the 'Range of Variability Approach' (RVA). The proposed approach derives from aquatic ecology theory concerning the critical role of hydrological variability, and associated characteristics of timing, frequency, duration, and rates of change, in sustaining aquatic ecosystems. The method is intended for application on rivers wherein the conservation of native aquatic biodiversity and protection of natural ecosystem functions are primary river management objectives. 2. The RVA uses as its starting point either measured or synthesized daily streamflow values from a period during which human perturbations to the hydrological regime were negligible. This streamflow record is then characterized using thirty-two different hydrological parameters, using methods defined in Richter et al. (1996). Using the RVA, a range of variation in each of the thirty-two parameters, e.g. the values at t 1 standard deviation from the mean or the twenty-fifth to seventy-fifth percentile range, are selected as initial flow management targets. 3. The RVA targets are intended to guide the design of river management strategies (e.g. reservoir operations rules, catchment restoration) that will lead to attainment of these targets on an annual basis. The RVA will enable river managers to define and adopt readily interim management targets before conclusive, long-term ecosystem research results are available. The RVA targets and management strategies should be adaptively refined as suggested by research results and as needed to sustain native aquatic ecosystem biodiversity and integrity.

How much water does a river need

Global biodiversity in river and riparian ecosystems is generated and maintained by geographic variation in stream processes and fluvial disturbance regimes, which largely reflect regional differences in climate and geology. Extensive construction of dams by humans has greatly dampened the seasonal and interannual streamflow variability of rivers, thereby altering natural dynamics in ecologically important flows on continental to global scales. The cumulative effects of modification to regional-scale environmental templates caused by dams is largely unexplored but of critical conservation importance. Here, we use 186 long-term streamflow records on intermediate-sized rivers across the continental United States to show that dams have homogenized the flow regimes on third- through seventh-order rivers in 16 historically distinctive hydrologic regions over the course of the 20th century. This regional homogenization occurs chiefly through modification of the magnitude and timing of ecologically critical high and low flows. For 317 undammed reference rivers, no evidence for homogenization was found, despite documented changes in regional precipitation over this period. With an estimated average density of one dam every 48 km of third- through seventh-order river channel in the United States, dams arguably have a continental scale effect of homogenizing regionally distinct environmental templates, thereby creating conditions that favor the spread of cosmopolitan, nonindigenous species at the expense of locally adapted native biota. Quantitative analyses such as ours provide the basis for conservation and management actions aimed at restoring and maintaining native biodiversity and ecosystem function and resilience for regionally distinct ecosystems at continental to global scales.

https://www.pnas.org/content/pnas/104/14/5732.full.pdf

Homogenization of regional river dynamics by dams and global biodiversity implications.

We define disturbance in stream ecosystems to be: any relatively discrete event in time that is characterized by a frequency, intensity, and severity outside a predictable range, and that disrupts ecosystem, community, or population structure and changes resources or the physical environment. Of the three major hypotheses relating disturbance to lotic community structure, the dynamic equilibrium hypothesis appears to be generally applicable, although specific studies support the intermediate disturbance hypothesis and the equilibrium model. Differences in disturbance frequency between lentic and lotic systems may explain why biotic interactions are more apparent in lakes than in streams. Responses to both natural and anthropogenic disturbances vary regionally, as illustrated by examples from the mid-continent, Pacific northwest, and southeastern United States. Based on a generalized framework of climatic-biogeochemical characteristics, two features are considered to be most significant in choosing streams...

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The role of disturbance in stream ecology.

The National Water-Quality Assessment Program of the U.S. Geological Survey is designed to assess the status of and trends in the quality of the Nation's groundand surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. The study design balances the unique assessment requirements of individual hydrologic systems with a nationally consistent design structure that incorporates a multiscale, interdisciplinary approach. The building blocks of the program are Study-Unit Investigations in 60 major hydrologic basins (Study Units) of the Nation. The Occurrence and Distribution Assessment is the largest and most important component of the first intensive study phase in each Study Unit. The goal of the Occurrence and Distribution Assessment is to characterize, in a nationally consistent manner, the broad-scale geographic and seasonal distributions of water-quality conditions in relation to major contaminant sources and background conditions. The national study design for surface water focuses on water-quality conditions in streams, using the following interrelated components: • Water-Column Studies assess physical and chemical characteristics, which include suspended sediment, major ions and metals, nutrients, organic carbon, and dissolved pesticides, and their relation to hydrologic conditions, sources, and transport. • Bed-Sediment and Tissue Studies assess trace elements and hydrophobic organic contaminants. • Ecological Studies evaluate the relations among physical, chemical, and biological characteristics of streams. Sampling designs for all three components rely on coordinated sampling of varying intensity and scope at Integrator Sites, which are chosen to represent water-quality conditions of streams with large basins that are often affected by complex combinations of land-use settings, and at Indicator Sites, which are chosen to represent water-quality conditions of streams associated with specific individual Environmental Settings. The national study design for ground water focuses on water-quality conditions in major aquifers, with emphasis on recently recharged ground water associated with present and recent human activities, by using the following components: • Study-Unit Surveys assess the water quality of the major aquifer systems of each Study Unit by sampling primarily existing wells. • Land-Use Studies use observation wells and selected existing wells to assess the quality of recently recharged shallow ground water associated with regionally extensive combinations of land use and hydrogeologic conditions. • Flowpath Studies use transects and groups of clustered, multilevel observation wells to examine specific relations among land-use practices, ground-water flow, and contaminant occurrence and transport and interactions between ground and surface water. In selected locations, ground-water studies are codesigned with streamwater-quality studies to investigate interactions between ground and

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Design of the National Water-Quality Assessment Program; occurrence and distribution of water-quality conditions

Stream habitat is characterized in the US Geological Survey’s National Water-Quality Assessment (NAWQA) Program as part of an integrated physical, chemical, and biological assessment of the Nation's water quality The goal of stream habitat characterization is to relate habitat to other physical, chemical, and biological factors that describe water-quality conditions To accomplish this goal, environmental settings are described at sites selected for water-quality assessment In addition, spatial and temporal patterns in habitat are examined at local, regional, and national scales This habitat protocol contains updated methods for evaluating habitat in NAWQA Study Units Revisions are based on lessons learned after 6 years of applying the original NAWQA habitat protocol to NAWQA Study Unit ecological surveys Similar to the original protocol, these revised methods for evaluating stream habitat are based on a spatially hierarchical framework that incorporates habitat data at basin, segment, reach, and microhabitat scales This framework provides a basis for national consistency in collection techniques while allowing flexibility in habitat assessment within individual Study Units Procedures are described for collecting habitat data at basin and segment scales; these procedures include use of geographic information system data bases, topographic maps, and aerial photographs Data collected at the reach scale include channel, bank, and riparian characteristics INTRODUCTION The US Geological Survey’s (USGS) National Water-Quality Assessment (NAWQA) Program is designed to assess the status of and trends in the Nation’s water quality (Gilliom and others, 1995) and to develop an understanding of the major factors that affect observed water-quality conditions and trends (Hirsch and others, 1988; Leahy and others, 1990) This assessment is accomplished by collecting physical, chemical, and biological data at sites that represent major natural and human factors (for example, ecoregion, land use, stream size, hydrology, and geology) that are thought to control water quality These data are used to provide an integrated assessment of water quality within selected environmental settings, assess trends in water quality, and investigate the influence of major natural and human factors on water quality Study Unit investigations in the NAWQA Program are done on a staggered time scale in approximately 59 of the largest and most significant hydrologic systems across the Nation (Gilliom and others, 1995) These investigations, which consist of 4 to 5 years of intensive assessment followed by 5 years of low-intensity assessment, consist of four main components—(1) retrospective analysis; (2) occurrence and distribution assessment; (3) assessment of long-term trends and changes; and (4) case studies of sources, transport, fate, and effects (Gilliom and others, 1995) Occurrence and distribution assessments are done in a nationally consistent and uniform manner for identification of spatial and temporal trends in water quality at a national scale (Gilliom and others, 1995) Characterization of stream habitat is an essential component of many water-quality assessment programs (Osborne and others, 1991) and an important element in the NAWQA Program (Gurtz, 1994)

/pdf/revised-methods-for-characterizing-stream-habitat-in-the-4ll60d9wbr.pdf

Revised Methods for Characterizing Stream Habitat in the National Water-Quality Assessment Program

Benthic invertebrate communities are evaluated as part of the ecological survey component of the U.S. Geological Survey's National Water-Quality Assessment Program. These biological data are collected along with physical and chemical data to assess water-quality conditions and to develop an understanding of the factors that affect water-quality conditions locally, regionally, and nationally. The objectives of benthic invertebrate community characterizations are to (1) develop for each site a list of tax a within the associated stream reach and (2) determine the structure of benthic invertebrate communities within selected habitats of that reach. A nationally consistent approach is used to achieve these objectives. This approach provides guidance on site, reach, and habitat selection and methods and equipment for qualitative multihabitat sampling and semi-quantitative single habitat sampling. Appropriate quality-assurance and quality-control guidelines are used to maximize the ability to analyze data within and among study units.

Martin E. Gurtz

Papers

The role of disturbance in stream ecology.

Design of the National Water-Quality Assessment Program; occurrence and distribution of water-quality conditions

Revised Methods for Characterizing Stream Habitat in the National Water-Quality Assessment Program

Methods for collecting benthic invertebrate samples as part of the National Water-Quality Assessment Program

Methods for sampling fish communities as part of the National Water-Quality Assessment Program