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Estimation of Peak Discharges
Introduction
A study of the magnitude and frequency of floods in Oregon has been completed by the Oregon Water Resources Department (OWRD) with financial assistance from the Federal Emergency Management Agency, Oregon Department of Transportation, and the Association of Oregon Counties and with the cooperation of the U.S. Geological Survey. The study was undertaken to provide engineers and land managers with the information needed to make informed decisions about development in or near watercourses.
 
Tools developed as a result of the study allow for the estimation of peak discharges at specified frequencies at most locations on most rural, unregulated streams in western Oregon.  The user is encouraged to read in detail the reports describing the study for western and eastern (PDF 9.7MB) Oregon.
 
It is the intent of the Oregon Water Resources Department that information regarding peak discharges generated by the tools provided here be evaluated and used by a qualified hydrologist familiar with the analysis and estimation of peak discharges.
 
Making Peak Discharge Estimates
Peak discharges are measured at a limited number of locations in Oregon. At these measurement sites (i.e., gaging stations), peak discharges at specified frequencies are estimated directly from the measurements. At unmeasured sites, peak discharges are estimated from prediction equations that are based on a regionalization of the measurements. In either case, the peaks are reported for recurrence intervals of 2, 5, 10, 20, 25, 50, 100, and 500 years.
 
All estimated peak discharges reported here represent stream flow unaffected by significant reservoir operations, diversions or urbanization. A number of the gaging stations used in this analysis are now affected by reservoir operations. The reported peak discharges represent the prior, unregulated condition of the watershed. The peak discharges reported for the Willamette River at Salem are an example.
 
Since the prediction equations are based on the measured peak discharges, they also do not account for reservoir operations, diversions or urbanization. Estimates made by either method for a watershed so affected represent a hypothetical condition of the watershed, not the actual condition. Making corrections for the effects of any of these conditions is the responsibility of the user.

Peak discharge estimates from the prediction equations
Peak discharge estimates from the prediction equations require estimates of several physical characteristics of the watershed of interest (e.g., area, average slope). Most of these characteristics are estimated from regionalized data. These data and their sources are described in the accompanying reports for western and eastern (PDF 9.7MB) Oregon.  Only the versions of the regionalized data described there should be used with the prediction equations.  Additionally, the best estimates of watershed characteristics are achieved by analyzing the regionalized data with geographic information systems technologies rather than manually making the estimates from plotted isoline maps.

For these reasons, making appropriate and reliable estimates of watershed characteristics may be difficult for many users. We strongly advise the user to take advantage of options 1, 3, and 4 listed below rather than estimating the watershed characteristics independently (option 2). All options below generate a report detailing peak discharges and how they were determined for the specified watershed.

Occasionally, the watershed above a selected site has characteristics outside the bounds of the characteristics of the gaging stations used to develop the prediction equations. The user will be warned when this occurs, and a list of the bounds on the watershed characteristics will be provided. Using the prediction equations with characteristics that are out of bounds is an extrapolation and may lead to poor results.
 
Peak discharge estimates from measurements at a gaging station
For a limited number of watersheds in Oregon, measurements of peak discharges have been made on a systematic basis. From these measurements, estimates of peak discharges at specified frequencies have been made. The estimates may be viewed by selecting the gaging station of interest from a list of the measured watersheds.  Get estimates of peak discharge at a gaging station​.

 
Unmeasured but already defined watersheds
The OWRD has delineated and estimated characteristics for over 3,000 watersheds in Oregon.  For these watersheds, estimates of peak discharge may be obtained by selecting a watershed from an interactive on-screen map.  Get estimates of peak discharges at an already defined watershed.

 
Manually entering the required characteristics
The user supplies the twelve watershed characteristics necessary to determine the peak discharge estimates. Again, making appropriate and reliable estimates of watershed characteristics may be difficult or inconvenient for many users. We advise against using this option.
 
Required Characteristics
  Drainage area square miles
Minimum watershed elevation feet
  Maximum watershed elevation feet
  Mean slope degrees
  Mean aspect degrees
  Mean elevation feet
Mean 2-year 24-hour precipitation intensity inches
Mean January precipitation inches
Mean July precipitation inches
Mean annual snowfall inches
  Mean January minimum temperature degrees Fahrenheit
Mean July minimum temperature degrees Fahrenheit
  Mean January maximum temperature degrees Fahrenheit
Mean July maximum temperature degrees Fahrenheit
Mean storage capacity inches
Mean soil permeability inches per hour
  Mean soil depth to bedrock inches
 
Get estimates of peak discharges by manually entering watershed characteristics.

 
Using the auto-delineation program
With this option the user selects a point on a stream where the peak discharge estimates are desired. Selection of the point is done interactively from topographic maps displayed on-screen. Nothing further is required from the user. Delineation of the watershed above the selected point, determination of the watershed characteristics, and calculation of the peak discharges are done automatically. Please remember that for large watersheds the auto-delineation could take up to an hour to complete.
 
This option uses a digital elevation model (DEM) as the basis for delineating the boundary of the watershed above the selected point. The method sometimes delineates the wrong watershed when the selected point is in a flat area, such as a valley floor, or near the confluence of two streams. The output from this option provides a map of the delineated watershed. The user should check the map carefully to be sure the correct watershed has been delineated. When the watershed is incorrect, moving the selected point slightly upstream or downstream usually yields an acceptable result. Occasionally, the method continues to fail regardless of the point selected. In these cases, it may be necessary to delineate the watershed by hand and submit the delineated watershed as a shape file (option 3). It also may be possible to find an already delineated watershed to use instead (option 1). The easiest way to find the point of interest is from its township, range and section. While it is possible to "zoom" in to the desired point, it may prove difficult to find. Get estimates of peak discharge using auto-delineation.