StreamStats in Oregon
Oregon StreamStats incorporates regression equations for estimating instantaneous peak flows with annual exceedance probabilities of 50, 20, 10, 4, 2, 1, and 0.002 percent in Western Oregon. These peak flows have recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years, respectively. Regression equations are not available for estimating peak flows in Eastern Oregon. Oregon StreamStats also now includes regression equations for estimating selected annual and monthly flow-duration and low-flow statistics. The flow-duration statistics include flows that are exceeded 5, 10, 25, 50, and 95 percent of the time. Low-flow statistics include the 7-day, 2- and 10-year flows. The reports below document the available regression equations for Oregon, the methods used to develop the equations and to measure the basin characteristics used as explanatory variables in the equations, and the errors associated with the estimates obtained from the equations:
- Cooper, R. M., 2005, Estimation of Peak Discharges for Rural, Unregulated Streams in Western Oregon: U. S. Geological Survey Scientific Investigations Report 2005-5116, 134 p.
- Risley, John, Stonewall, Adam, and Haluska, Tana, 2008, Estimating flow-duration and low-flow frequency statistics for unregulated streams in Oregon: U.S. Geological Survey Scientific Investigations Report 2008-5126, 22 p.
Users should familiarize themselves with the above reports before using Oregon StreamStats to obtain estimates of streamflow statistics for ungaged sites. The peak-flow equations from the report by Cooper are applicable only in Western Oregon, west of the crest of the Cascades Mountains; however, the equations by Risley and others are available State-wide. When zoomed into areas that east of the Cascades Mountain crest, the StreamStats map will display hatching over the base map as an indicator that the peak-flow equations are not available there. This hatching can be turned off by clicking on the check box for the Area of limited functionality layer in the OR Map Layers frame.
Note to Users of Oregon StreamStats on Handling of Estimates for Flood Region 2
Flood Region 2 is subdivided into Regions 2A and 2B. Region 2A includes watersheds with mean basin elevations greater than or equal to 3,000 feet and Region 2B includes watersheds with mean basin elevations less than 3,000 feet. USGS Scientific Investigation Report (SIR) 2005-5116 (equation 8, page 44) suggests a weighting method to determine flood-frequency estimates for ungaged sites in a transition zone of mean basin elevations between 2,875 and 3,125 feet:
QT = Q2b((3,125-E)/250) + Q2a((E-2,875)/250)
where QT, Q2a, and Q2b are the weighted discharge for sites in the transition zone, the discharge estimated from the Region 2A equation, and the discharge for Region 2B, respectively, and E is the mean basin elevation.
StreamStats is implemented such that:
- If the mean basin elevation for the ungaged site is greater than 2,875 feet and less than 3,125 feet StreamStats output will provide estimates based on both sets of equations, as well as the weighted estimate as calculated by equation 8, USGS SIR 2005-5116.
- If the mean basin elevation for the ungaged site is less than 2,875 feet StreamStats output will provide estimates based on the equations for Region 2B only.
- If the mean basin elevation for the ungaged site is greater than 3,125 feet StreamStats output will provide estimates based on the equations for Region 2A only.
Three rivers, the Nehalem, Umpqua, and Rogue, flow from Region 2 through Region 1, to the Pacific Ocean. Watersheds delineated on these rivers in Region 1 will include upstream areas in Region 2. In these situations, the flow estimates are computed using regression equations for both Regions 1 and 2. First the flows are computed using the characteristics of the full watershed with the Region 1 equations. These flows are given under the table heading "Streamflow Statistics Reg 1 Coastal Cooper". Second, the flows are computed using the characteristics of the full watershed with the Region 2 equations as described above. These flows are given under the table heading "Streamflow Statistics Reg 2 Western Interior GE (and/or LT) 3000 ft Cooper". Then finally, an area-weighted average of the flows is computed using the areas in Regions 1 and 2. These flows are given under the table heading "Streamflow Statistics Elevation- and/or Area-Averaged.
Only the basin characteristics required for use in the peak-flow equations for the region(s) in which the basin for the ungaged site is located are listed in the report produced by the FlowStats tool; however, a suite of basin characteristics are available in the report produced by the BasinChar tool.