5.2B – Managing Hydromodification Impacts – Flow and Temperature Maintenance

Management Measure

  1. Maintain or restore hydrologic stream flows that promote stream channel equilibrium and are appropriate (using the river continuum concept) for the watershed.
  2. Use modeling to determine the effects of stream channel and riparian management activities (including reservoir release operations) on water temperatures and flows.

Water quality and quantity are interrelated. Stream flow affects surface water temperature and turbidity which in turn affects dissolved oxygen concentrations. Stream flow has an obvious effect on mixing and concentration of hydrophilic chemicals and salts. The solubility for many chemicals is increased along with increasing temperature. Therefore, in addition to metabolic stress, increased water temperatures can also cause aquatic organisms to suffer from toxic stress. Stream flow also transports sediments, nutrients, and pollutants. In areas where flow is restricted, stagnant water can accumulate sediments, nutrients, and pollutants. Accumulation of sediment and nutrients in waterbodies is a positive feedback loop that increases water temperature and nutrients, specifically phosphorus concentrations, which reduces dissolved oxygen and promotes algae blooms - which further reduces dissolved oxygen. In extreme cases this process may cause eutrophication and/or harmful algae blooms.

Water quantity and ecosystem functions are interrelated. High flows are cues for some fish to migrate and/or spawn. High flow events clean and sort sediment by scouring and depositing stream bed gravels and course-grained sediment. Clean, well aerated and sorted gravels are important for successful salmon spawning. Seasonal high flows assist in the dispersal and germination of some riparian species, such as cottonwoods and willows. Channel-forming flows effect sediment erosion, dispersal and deposition which create mosaics or patches of diverse wetland and riparian plant communities. The spatial heterogeneity of these plant community mosaics is an important component of ensuring ecological resilience.

Management Practices


  • Maintain or restore the pre-development hydrograph and/or maintain stream channel equilibrium by determining the stream flow characteristics for your watershed. Use the river continuum concept to determine the appropriate flow management regime for your site.
  • Maintain channel geometry and channel bed form so that flow characteristics are somewhat complex and mimic natural conditions, as appropriate for your watershed. Use the river river continuum concept to determine the appropriate stream channel geometry for your site.
  • Consider using flow augmentation if your site is downstream of an operating dam. Flow augmentation is a high-magnitude, short-duration reservoir release for the purpose of maintaining channel capacity and improving the quality of aquatic habitat for salmon spawning by scouring the accumulation of fine-grained sediments from the course-grained streambed. Refer to MM 5.1C for more information on Dam Operations and Maintenance.

Peak and Low Flow Frequency Analysis

Methods to determine the stream flow frequencies for peak and low flow conditions include qualitative, quantitative and estimated (semi-quantitative).

Qualitative methods include determining what type of stream (perennial, intermittent, or ephemeral); and the relative contributions of baseflow (groundwater) vs. stormflow (runoff) comprise the annual water budget of your stream and its watershed.

  • Perennial streams flow continuously (during both wet and dry seasons) due to a constant source of groundwater inputs or baseflow.
  • Intermittent streams have the potential for continuous flow but are heavily reliant on seasonal precipitation events (for both baseflow and stormflow) which last at least 30 days per year.
  • Ephemeral streams flow only during or immediately after periods of precipitation which is generally less than 30 days per year.

Common quantitative methods used to determine the relative frequency and duration of high and low flow events in a stream include, flood frequency analysis and the production of a flow duration curve. The Interagency Advisory Committee on Water Data (1982) provides a standard procedure for peak-flow frequency calculation that involves a standard frequency distribution the log-Pearson Type III (LPIII) distribution. Low flow frequency analysis is less standardized. One approach is to use only low rainfall months (late summer to early fall) to determine the annual minimum 7-day and 10-year low-flow event.

Common semi-quantitative or estimated methods used to determine the flow characteristics of streams include indirect methods. A common indirect method is regional hydrologic analysis which provides estimates of mean annual flow and flood characteristics, but does not give any information on low flow events. Due to the spatial scale of empirical data used for this method, it is often invalid for small watersheds (less than 31 square miles or 80 square kilometers). If you are working in a small, watershed, calibrate regional data (regional curves) with actual instantaneous flow data from your watershed. Also, be wary of using historical data for highly modified watersheds, stream flow characteristics change as a result of changing watershed management activities. Other types of indirect stream flow measurements include the use of models based on canonical correlation analysis and artificial neural networks.

Channel-forming or dominant discharge flows are a function of streamflow frequency and volume.

Wolman and Miller (1960) found that 90% of the sediment load is transported by storm events that occur with a frequency of 5 year or less, of these, moderate-frequency storms (reoccurrence interval of every few years) cause the river to carry approximately 50% of that sediment load. A common method to determine the channel-forming flow uses field indicators to determine the bankfull discharge. The bankfull discharge is based on a regression relationship between the minimum cross sectional area (width of the channel multiplied by the average depth of the stream) vs. the watershed area. Bankfull width is determined by the first depositional surface, other indicators of bankfull width include changes in vegetation. Increase your confidence in bankfull width by using multiple indicators. Bankfull width determination is subjective and difficult to determine; therefore it should only be done by a professional on streams that are in equilibrium (sediment deposition and transport is in balance with flows appropriate for the watershed).

Effective discharge is a calculated value that represents an increment of discharge that transports the largest fraction of sediment load (sediment discharge rating curve) over a defined frequency.

Mean annual flow can be used to determine a channel forming flow. Typically this value is less than bankfull discharge.


  • Manage water quantity so that minimum stream flows provide adequate water temperature for the designated beneficial uses. See MM 5.1C for more information on managing reservoir releases that provide temperature ranges necessary to support aquatic life downstream of operating dams.
  • Plant and maintain riparian vegetation, such as native trees, that provide shade to small creeks and streams. Make sure that the soil and hydrologic regime along the stream banks where riparian vegetation is planted will support it. For example, if germination and natural recruitment of riparian vegetation is not occurring at the planting site, then it is possible that management activities such as soil nutrient replenishment and irrigation will be necessary (especially during seedling establishment phase) to increase the survival of planted riparian vegetation. Riparian vegetation planted along incised streams may need longer irrigation intervals to ensure establishment because the roots will need to grow deeper to access groundwater.


  • Marin County Stormwater Pollution Prevention Program has programs and resources for targeted audiences within the Marin County area all available on their easy to navigate Website.
  • Federal Emergency Management Agency (FEMA) is part of the U.S. Department of Homeland Security (DHS). The primary mission of FEMA is to reduce the loss of life and property and protect the Nation from all hazards, including natural disasters, acts of terrorism, and other man-made disasters, by leading and supporting the Nation in a risk-based, comprehensive emergency management system of preparedness, protection, response, recovery, and mitigation. FEMA certifies maps designating 100 and 200 year floodplains.
  • California Tahoe Conservancy has undertaken a comprehensive program to reduce the sources of soil erosion and the amount of sediment and algae-encouraging nutrients that reach Lake Tahoe.
  • Department of Water Resources (DWR) FloodSAFE has five main goals, 1) to increase flood protection, 2) to improve preparedness and response to flooding, 3) to support a growing economy, 4) to enhance ecosystems, and 5) to promote sustainability.
  • Sonoma County Water Agency's Stream Maintenance Program Manual.
  • US Army Corps of Engineers (USACE), Water Resources is overseen by the Directorate of Civil Works. Funds for the Civil Works program come from the annual Energy and Water Development Appropriation, not the Defense budget, many projects also receive cost-sharing funds supplied directly by non-Federal sponsors for specific projects. Activities that are funded include projects relating to improving and maintaining navigable waterways, flood damage reduction, hydroelectric power generation, shore protection and recreation.
  • US Bureau of Reclamation, Water Measurement Manual (USDI-BOR 2001) purpose is to provide water users and districts guidance in selecting, managing, inspecting, and maintaining their water measurement devices. The second is to describe the standard methods and devices commonly used to measure irrigation water. The third is to acquaint irrigation system operators with a variety of other established but less common methods and with new or special techniques. This manual provides information for measuring stream channel geometry and discharge rates.

Information Resources

  • ColSIM is a modular simulation environment, primarily designed for development of controllers in thermal systems (flood control and power demand). High calculation performance with a time horizon of a whole year using timesteps down to some seconds is achieved. This open source (ANSI-C) script is maintained by Christof Wittwer at Fraunhofer Institute for Solar Energy Systems in Freiburg/Germany.
  • QUAL2K is a river and stream water quality model that is programmed in the Windows macro language: Visual Basic for Applications (VBA). Excel is used as the graphical user interface.
  • FLOW-3D is a modeling tool that gives engineers valuable insight into many physical flow processes. With special capabilities for accurately predicting free-surface flows, FLOW-3D is the ideal software to use in your design phase as well as in improving production processes.
  • Hydrologic Engineering Center (HEC), an organization within the Institute for Water Resources, is the designated Center of Expertise for the US Army Corps of Engineers in the technical areas of surface and groundwater hydrology, river hydraulics and sediment transport, hydrologic statistics and risk analysis, reservoir system analysis, planning analysis, real-time water control management and a number of other closely associated technical subjects. HEC supports Corps field offices, headquarters, and laboratories by providing technical methods and guidance, water resources models and associated utilities, training and workshops, accomplishing research and development, and performing technical assistance and special projects. The products that are developed from these activities are for the Corps but are available to the public and may be freely downloaded from this Website.
  • BASMAA, Best Management Practices (BMP) for Routine Flood Control Maintenance Activities, is a desk reference that describes and illustrates appropriate flood control BMPs.
  • Cleland, B. 2002. TMDL Development from the "Bottom Up" - Part II: Using Duration Curves to Connect the Pieces. Americas Clean Water Foundation. Describes how the use of flow duration curves can help identify targeted areas, targeted programs, targeted activities, and targeted participants in developing NPS related TMDLs.
  • DWR, CALSIM is a generalized water resources simulation model for evaluating operational alternatives of large, complex river basins. Developed by the Department of Water Resources, CALSIM integrates a simulation language for flexible operational criteria specification, a linear programming solver for efficient water allocation decisions, and graphics capabilities for ease of use. These combined capabilities provide a comprehensive and powerful modeling tool for water resource systems simulation.
  • Niadas, I.A. 2004. Regional flow duration curve estimation in small ungauged catchments using instantaneous flow measurements and a censored data approach.
  • NRCS, National Water Management Center (NWMC) is working with other Federal, State, and Local agencies to develop Regional Hydraulic Geometry Curves across the country. Regional Hydraulic Geometry Curves are log-log plots comparing channel dimensions (top width, mean depth, and cross-sectional area) at 'bankfull' or effective discharge (usually between the 1.1 and 1.9 year return interval) versus drainage area.
  • Shu, C. and T. Ouarda. 2007. Flood frequency analysis at ungauged sites using artificial neural networks in canonical correlation analysis physiographic space. Water Resources Research 43: W07438, doi:10.1029/2006WR005142. Discusses the use of indirect stream flow measurements using models based on canonical correlation analysis (CCA) and artificial neural networks (ANNs).
  • USACE, Hydrologic Frequency Analysis contains technical information that can be used to determine flood frequency parameters.
  • USGS, San Francisco Bay Hydrodynamics Project, conducts hydrodynamic transport investigations, in collaboration with a broad coalition of state and federal agencies (DWR, SWRCB, DFG, USBR, and USFWS), by using a combination of three components: Delta Flows Monitoring, Process-Based Field studies and Three-dimensional Modeling.
  • USGS, Techniques and Methods 4-A6: A Computer Program for Estimating Streamflow Statistics for Ungauged Sites. This report describes the regionalization techniques used to develop the equations in NSS and provides guidance on the applicability and limitations of the techniques. The report also includes a users’ manual and a summary of equations available for estimating basin lagtime, which is needed by the program to generate flood hydrographs. The NSS software and accompanying database, and the documentation for the regression equations included in NSS, are available on the Web at http://water.usgs.gov/software/.
  • USGS, Program PeakFQ provides estimates of instantaneous annual-maximum peak flows for a range of recurrence intervals, including 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200, and 500 years (annual-Exceedance probabilities of 0.6667, 0.50, 0.4292 0.20, 0.10, 0.04, 0.02, 0.01, 0.005, and 0.002, respectively). The Pearson Type III frequency distribution is fit to the logarithms of instantaneous annual peak flows following Bulletin 17B guidelines of the Interagency Advisory Committee on Water Data. The parameters of the Pearson Type III frequency curve are estimated by the logarithmic sample moments (mean, standard deviation, and coefficient of skewness) with adjustments for low outliers, high outliers, historic peaks, and generalized skew.
  • USGS, Stream Network and Stream Segment Temperature Models Software (SNTEMP) is a mechanistic, one-dimensional heat transport model that predicts the daily mean and maximum water temperatures as a function of stream distance and environmental heat flux. Net heat flux is calculated as the sum of heat to or from long-wave atmospheric radiation, direct short-wave solar radiation, convection, conduction, evaporation, streamside vegetation (shading), streambed fluid friction, and the water's back radiation. The heat flux model includes the incorporation of groundwater influx. The heat transport model is based on the dynamic temperature-steady flow equation and assumes that all input data, including meteorological and hydrological variables, can be represented by 24-hour averages. This model was used for several sites and the results are included in Scientific Investigations Report 2008-5070.
  • USGS, Studley, S. Water-Resources Investigations Report 00-4113, Estimated Flow-Duration Curves for Selected Ungauged Sites in the Cimarron and Lower Arkansas River Basins in Kansas. This paper shows how estimated flow-duration curves at the ungauged sites can be used for projecting future flow frequencies for assessment of total maximum daily loads (TMDL's) or other water-quality constituents and for water-availability studies.


USEPA. 2007. National Management Measures Guidance to Control Nonpoint Source Pollution from Hydromodification. EPA 841-B-07-002. U.S. Environmental Protection Agency, Washington, DC.

Wolman, M.G. and J.P. Miller. 1960. Magnitude and Frequency for forces in geomorphic process. Journal of Geology 68: 54-74.

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