SMP Nuts and Bolts

Assess Conditions and Identify Risks

Streamflow, which is strongly correlated with many critical physicochemical characteristics of rivers…, can be considered a “master variable” that limits the distribution and abundance of riverine species and regulates the ecological integrity of flowing water systems.

Poff, et al. 1997

A river’s natural flow regime is determined by precipitation and watershed characteristics. However, human activities may alter the natural flow regime through water management (e.g., withdrawals, augmentations), storage, and land use changes.

Most, if not all, stream management planning processes devote a significant amount of time and effort to understanding the hydrology of their river. Details about this often-complex analysis, including data types, data sources, analysis techniques, and tips are available in the paragraphs below.

Depending on the objectives of a particular SMP/IWMP, calculation of minimum environmental flows required to support resident fish populations may also be considered. Information and methodologies related to minimum flow evaluations are available here.

Alterations to the magnitude, duration, frequency, timing, and rate of change of natural flow patterns are important to sustain human uses of water and the life and livelihoods that depend on them. The goal of a SMP is rarely to return to pre-human flow patterns. However, changes in flow patterns can cause significant impacts that SMPs can assess and seek to minimize. Flow alterations can impact numerous aspects of riverine ecosystem structure and function, including aquatic biota at different life stages, channel form, riparian vegetation, floodplain functions, water quality, and aquatic habitat availability.

Flow regime is represented by graphs of flow over time (hydrographs) and flow duration curves that show the timing, magnitude, and frequency of flow conditions. Alterations to the flow regime can be characterized by statistical variability or visual comparison of hydrographs and flow duration curves.

Click to Expand Graphic

From Van Seeter, MM and J Pitlick. 1998. Geomorphology and endangered fish habitats of the Upper Colorado River: historic changes in streamflow, sediment load, and channel morphology. Water Resources Research 34:2, p. 287-302.

The geographic scope and goals of an SMP, as well as the amount of existing data, should influence the assessment level, modeling methods, field verification, and data analysis methods selected to characterize flow conditions. Understanding the temporal and spatial scale of the data needed to adequately characterize conditions or run scenarios is best done early in the process so additional hydrologic data can be collected if needed.

Data Types

Total Volume

Peak flow and its duration and timing

Low flow/baseflow and its duration and timing

Flow variability

Rate of change

Zero flow days

Potential Data Sources

CDSS models (StateMod water allocation and accounting model)

Regional regression equations for estimation of streamflow

Literature reviews

Interviews with local residents

Tips for Success


The best tool to understand the drivers behind streamflow (water rights, return flows, etc.), or to model streamflow at points between gauges, is Colorado’s integrated system of data and simulation models, the Colorado Decision Support System (CDSS). CDSS consists of several components including:

  1. a database of hydrologic and administrative (water right and diversion) information called HydroBase;
  2. a variety of tools for reviewing, reporting, and analyzing data; and
  3. models to simulate water consumption and the operation of water diversions, storage, use, and return flows.

Anyone can access CDSS; however, it is a complex system of tools that requires specialized training to fully utilize. Colorado has many hydrologists and water resource engineers who can perform such work.


In addition to estimating flows in between gauges, CDSS (specifically, the surface water modeling component called StateMod) can project flows associated with future changes. If an SMP wants to investigate flow changes due to construction of new water storage facilities, change of water rights from agricultural to municipal uses, or increases in agricultural water use efficiency from projects like ditch lining or piping, StateMod can simulate those scenarios. Exercise caution, however, because StateMod’s baseline output assumes that all ditches are operational and all water users intend to fully irrigate their cropped acres. This maximizes water use to identify legal and physical water shortages, but may not accurately characterize current conditions. StateMod is a either a daily or a monthly modeling platform, but the models developed by the State of Colorado are monthly datasets and do not provide daily statistics. They can be used to develop a daily model, but this requires a large effort. A daily StateMod model may still not accurately reflect actual conditions (e.g., dry up locations).


Many parts of Colorado do not have sufficient streamflow data or diversion records to accurately represent daily conditions. There may be insufficient measurement points or a lack of automatic recording devices. Numerous tributaries and diversions may not be recorded on a daily, or even weekly, basis. While records may be accurate on the monthly or annual basis, daily conditions—important for aquatic habitat and fish spawning—can be less accurate. Short-term, daily data collection efforts as part of an SMP can help inform standard estimation techniques, but should not be given the same weight as long-term data. For example, if an ungauged tributary is an important piece of the SMP, a temporary stream gauge can be installed for three years to help calibrate daily streamflow estimates. These estimates can be useful for the SMP, but should be considered in the context of long-term daily data records elsewhere in the basin. Additionally, the short-term gauging efforts will not capture the full range of streamflow variability. This means that estimates may be biased by the small sample of year-types that were observed. If the basin has no long-term daily records, a streamflow-based method is not appropriate. Understanding the relationship between modeled hydrology and on-the-ground conditions is important so that questions asked of the model, or the model used, are aligned. Remember that models are only as good as the data provided to them.


In watersheds where hydrologic data is insufficient to answer questions, stakeholders can prioritize areas of concern or interest. In those areas, anecdotal or qualitative information, in conjunction with quantitative fieldwork, can identify problem areas or characterize the flow regime with a higher degree of accuracy. Fieldwork may consist of surveying cross sections at reaches of interest and using 2-D modeling to determine water availability at a range of flows.


The 2019 Colorado Environmental Flow Tool was designed to serve as a resource to help Basin Roundtables refine, categorize, and prioritize their portfolio of environmental and recreational (E&R) projects and methods through an improved understanding of flow needs and potential flow impairments, both existing and projected. The Flow Tool uses hydrologic data from Colorado’s Decision Support System (CDSS), additional modeled hydrologic data for various planning scenarios, and established flow-ecology relationships to assess risks to flows and E&R attribute categories at pre-selected gages across the state. The Flow Tool is a high-level tool that is intended to provide guidance during development of Stream Management Plans and Basin Implementation Plans (BIP).


If significant hydrologic data are missing, an SMP may need to rely on standard estimation techniques, which can be improved with short-term data collection efforts, but are still more uncertain than long-term data.


Hydrology Modeling

Crystal River Management Plan

Assessment Level: Level 3

An Ecological Decision Support System (EcoDSS) was developed to understand historical and current flow regime characteristics along the Crystal River. The EcoDSS is a collection of loosely coupled hydrological, hydraulic, and ecosystem response models that jointly simulate and predict the impact of water use and channel structure on stream hydrology and ecology using widely available data. The hydrologic modeling component of the EcoDSS provided 30 years of daily hydrologic simulation results for reaches of the Crystal River where management activities impact the flow regime. Results encompassed a range of drought and flood conditions and modeled patterns of flow withdrawal for municipal and agricultural uses. Running the EcoDSS with all of the surface water diversions switched “off” provided a simulation dataset approximating natural conditions. The EcoDSS quantitatively assessed the impact that different management practices have on total annual water yield, magnitude and duration of peak and base flows, variability of peak flow frequency and timing, and rates of hydrograph recession. The functional condition of the flow regime was determined by a set of “indicators of hydrologic alteration” metrics (such as median annual flow, median August flow, date of maximum flow, base flow index, rates of change).

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Big Thompson River Envisioning Project and Stream Management Plan

Assessment Level: Level 1

For the Big Thompson River SMP, the Big Thompson Watershed Coalition (BTWC) worked with project partner Northern Colorado Water Conservancy District (Northern Water) to understand basin hydrology. Northern Water maintains a number of point flow models of the South Platte River and its northern tributaries. The point flow models use historical gage and diversion data to calculate un-gaged gains and losses along each river reach. The resulting model output contains calculated river flows above and below each gage, inflow, and diversion point. This model output allows for historical hydrologic characterization of flows at many points along the river. As part of the Big Thompson SMP, BTWC’s consultant team synthesized the historical data to provide an evaluation of how river flows are currently managed, and may also assess how projected future water demand scenarios could alter management strategies.  

Cities, towns, and water utilities often develop customized hydrology models based on CDSS and StateMod but augmented with additional data in their use area. If such a model has already been developed encompassing your study area, it can be a useful tool. However, the use of these proprietary models may incur significant time and resource delays, so plan ahead if this type of model may suit your needs. For example, the Eagle River IWMP plans to use a proprietary model developed by the Eagle River Water and Sanitation District for modeling the basin’s streamflows, but they are experiencing lengthy delays while negotiating permission to use this model in their IWMP and are considering other options.

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Saint Vrain and Left Hand Stream Management Plan

Assessment Level: Level 1

The Saint Vrain and Left Hand Creek Stream Management Plan is leveraging the South Platte Decision Support System (SPDSS) in combination with an existing StateMOD model to assess existing water supply and allocation for various water users in the basin, including agricultural, municipal, and recreational. These models are being used to quantify baseline water gaps for water users in basin.

To assess existing agricultural water gaps in the basin, a StateCU model was developed to quantify the basin dry-year and average-year agricultural Irrigation Water Requirement (IWR). Water shortage estimates are then quantified by subtracting the IWR from the modeled available water on a structure basis. Irrigation efficiencies are also considered as part of this process. Climate change and its impacts to agricultural water users may also be assessed using the StateCU model by quantifying changes in the basin-wide IWR as a result of predicted regional temperature and precipitation change.

Municipal demands are quantified using existing water supply planning documentation available from each municipality in the basin. A literature review of available water supply planning documentation is serving as the primary basis to evaluate existing and future municipal water supply needs as part of this SMP.

In-stream recreational needs, assessed by American Whitewater (AW), will be integrated into the StateMOD model to evaluate recreational flow water gaps. Utilizing AW’s library of Recreational Flow Studies for basins across Colorado is recommend for quantifying recreational water gaps.

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Minimum Flow Evaluations

Evaluations of minimum flows necessary to support resident fish and other aquatic biota may be warranted for your SMP. Several methodologies may be applied to help you explore, collect, and analyze data related to minimum flows and biologically critical thresholds. These approaches range from relatively simple to quite complex, and have different spatial resolution, data needs, and costs associated with them. Review this brief summary to help you decide which is the most appropriate method for your circumstances.

Tennant Method – The Tennant Method, also referred to as the “Montana Method,” is a simple desktop method that is widely applied for setting in-stream flows. Self-described as a “quick, easy methodology… for determining flows to protect the aquatic resources in both warmwater and coldwater streams,” this method is based on calculating a percentage of average annual flows to determine biologically critical thresholds for both summer and winter months. The methodology is detailed in this paper (Tennant 1976).

R2Cross – The R2Cross method is based on a hydraulic model and uses field data collected in a stream riffle. Field data necessary to implement this method include streamflow measurements, cross-sectional and longitudinal surveys of channel geometry, and pebble counts to determine grain size distribution. The field data are then used to model three hydraulic parameters: average depth, average velocity, and percent wetted perimeter. Maintaining these hydraulic parameters at adequate levels across riffle habitat types also will maintain aquatic habitat in pools and runs for most life stages of fish and aquatic macro-invertebrates (Nehring 1979). The summer flow recommendation is based on meeting all three of the hydraulic criteria, and the winter flow recommendation is based on meeting two of the three criteria. CWCB’s R2Cross Manual (Espegren 1996) is provided here, and an updated R2Cross analysis program hosted on CSU’s eRAMS platform can be accessed here.

Physical Habitat Simulation (PHABSIM) – The PHABSIM system is a software product developed by USGS to simulate a relationship between streamflow and physical habitat for various life stages of a species of fish or a recreational activity of interest in order to improve the ability to make measurable tradeoffs between the various uses. The two basic components of PHABSIM are the hydraulic and habitat simulations of a stream reach using defined hydraulic parameters and habitat suitability criteria. Hydraulic simulation is used to describe the area of a stream having various combinations of depth, velocity, and channel index as a function of flow. This information is used to calculate a habitat measure called Weighted Usable Area (WUA) for the stream segment from suitability information based on field sampling of the species of interest. Simulation of physical habitat is accomplished using the physical structure of the stream and streamflow. The modification of physical habitat by temperature and water quality is analyzed separately from the physical habitat simulation contained in PHABSIM (USGS 2012). PHABSIM software can be downloaded here.

The Instream Flow Incremental Methodology (IFIM) provides a framework for applying PHABSIM in a water resource decision setting.

River-2D – River-2D is a two-dimensional hydraulic habitat model that has been customized for fish habitat evaluation studies (Steffler 2000). The River2D model simulates hydraulic conditions in natural rivers from topographic data input, and uses habitat suitability curves containing known biological preference data to calculate the potential habitat for specific species life-history stages by obtaining the Weighted Usable Area (WUA) for that species.

South Boulder Creek SMP

The South Boulder Creek SMP conducted a flow assessment in Phase 1 of their process to examine whether results of past minimum flow analyses are still relevant following the significant flood event in 2013. The team performed an R2Cross assessment along South Boulder Creek and compared the results of that assessment to previous CDOW R2Cross studies conducted since 1980. Appendix E of the South Boulder Creek SMP Phase 1 Final Report provides more information on their procedure.

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Grand County SMP (Upper Colorado)

The Grand County SMP used PHABSIM methods to investigate the relationships between streamflow and available trout habitat in 21 of the 30 Grand County SMP reaches. Flow-habitat relations developed for each study site, target species (mainly brown and rainbow trout), and life stage (adult, juvenile, spawning, incubation, winter survival) were examined to determine the recommended environmental target flow ranges for both summer and winter seasons.

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  1. This overview of Colorado’s Decision Support System lays out the basic components and types of data available
  2. CO Basin Data Dashboards
  3. Crystal River EcoDSS

Explore other ecological variables

Colorado SMP Library