Examples: Water Supply Capacity of a Small Stream, North Fork Yachats River

Location:  The Yachats River is located in the Mid Coast of Oregon.  The river flows southwest.

Description of Project:  Assume that three water rights permit applications are under review for a combined 7 cfs to be issued to (1) the city of Yachats and (2) two private landowners.  Assume that the city applicant is to receive 3 cfs while the two landowner applicants are to receive 2 cfs each.  Further, assume that there is a minimum 2 cfs instream flow requirement for fish habitat and passage. The diversion for all three applicants is to be placed on the North Fork of the Yachats River two kilometers north of the North Fork/Mainstem confluence.  The Yachats is an ungaged river.

Objectives:  To determine during what portion of the year the diversion of 7 cfs can be achieved while maintaining an instream flow of at least 2 cfs.

Plan of Action:  Compare nearby gaged systems to choose a data set to perform the hydrologic analyses for this project.  Determine how much flow is passed per season using a monthly analysis.  Develop a flow duration curve based on mean daily discharge data to know what percent of time flow will be at or below 2 cfs (no water available for diversion).  Ascertain the number of months that have days with flow at or below 2 cfs using mean monthly discharge averaged over the entire period of record.  Construct flow duration curves on a daily basis for critical months to determine the percent of time flow can and cannot be diverted during each critical period.

As you read through this example, you may wish to follow along with the analysis steps in an MS Excel file. You can download the data file by clicking here.

Step 1:  Delineate the watershed and determine the drainage area of the North Fork Yachats River using topographic maps.

Topographic maps used in this example are published by the U.S. Geologic Survey (USGS) and were obtained for use here from the online map source, topozone.com.  The watershed was delineated using identifiable ridges where possible.  For the areas of the watershed where the ridgetops are not clearly identified, headwaters of streams in adjacent watersheds were used to determine the correct location of the drainage divide.  For this example, drainage area was calculated using an overlay grid of boxes with known area.  The boxes were counted and the number of boxes within the watershed multiplied the area of an individual box.  The watershed drainage area was found to be 10 mi2.

Step 2:  Review the preliminary estimations page to determine a rough estimate of streamflow and precipitation values in this region.

The preliminary estimations for the North Coast will appear as follows:


Range for Annual Precipitation

70 – 160 in

1778 – 4064 mm

Annual discharge per unit area

5.17 ft3/mi2

0.057 m3/km2

Monthly flow as a percent of Annual Flow (%)

























Note that the values for monthly flow as a percentage of annual flow do not add up to 100%.  This is due to overlapping of drainage areas (i.e., nesting) of some or all gages used to calculate the percentages. 

The preliminary estimations for this basin show that you can expect the annual rainfall to be approximately 70 inches.  The lower value of the range was chosen because the study site is at an elevation that would not be influenced by orographic lift experienced higher in the mountain range.  Had the study site been located at or near the headwaters of the stream, the upper value would have been chosen.  With a drainage area of 10 mi2, the annual discharge is expected to be approximately 52 cfs.  The flow regime for a typical water year is anticipated to follow the general trend for west of the Cascades, namely, low flow during the summer months, with peak flows occurring during the winter months.  Furthermore, the most days of inundation are expected to occur during the months of December, January, and February.

Step 3:  Identify and list the characteristics of all nearby gages.

A review of the Table of USGS gages for the mid coast of Oregon shows three gages that may be appropriate choices for data in which to perform the hydrologic analysis of North Fork Yachats River: North Fork Alsea, Five Rivers, and Big Creek. 

Station Number

Station Name



Number of Water Years

Drainage Area
  (sq mi)

Altitude (ft above sea level)

Mean Annual Discharge (cfs)

Discharge/Unit Area
(cfs/sq mi)





























It is advantageous to select a gaged river that has a long period of record.  The table shows that the North Fork Alsea, Five Rivers, and Big Creek have 31, 30, and 19 years in the period of record, respectively.  Thus far, North Fork Alsea and Five Rivers are potential candidates for sources of data.  However, drainage area is another vital factor to consider.  If the drainage area of the gaged stream is similar to that of the study stream, then the uncertainty involved with scaling up or down can be avoided.  Observing the table, North Fork Alsea, Five Rivers, and Big Creek have 63, 114, and 12 mi2 of drainage area, respectively.  If all three rivers exhibit the same flow regime, then the Big Creek gage is the one to choose since it has such a comparable drainage area to that for the North Fork of the Yachats (10 mi2).  The problem with the shorter period of record can be solved through a validation procedure using normalized data from the long period of record of North Fork Alsea.  Nevertheless, it is wise to perform further analyses comparing the data sets for more than one gage before making a final decision.

Step 4:  Perform simple statistics on data to choose the most appropriate gage.

The general pattern of streamflow for each river can be viewed by plotting mean monthly discharge versus time for the period of record.  To compare the patterns of more than one system, normalization of the data is done by dividing each value by the drainage area of the respective system.  As can be seen in the figure, the patterns for all three systems are similar.  The drastic differences seen between Big Creek and the other two systems could be explained by basin geology (bedrock types), infiltration rates, or differences in annual precipitation due to topographical influence. The figure also emphasizes the substantial difference in the size of the drainage area of Big Creek versus the two rivers; nonetheless, the important item to note is the similar pattern. 

Plotting mean monthly discharge normalized by mean annual discharge for a typical water year, shows that the pattern still holds, but in addition to this validation, it also suggests slight dissimilarities between the three sets of data.  It appears that Big Creek and Five Rivers peak early while only Big Creek maintains a higher base flow during the dry months.  The discrepancy could also have developed due to gaps (missing years) in the period of record for one or all gages.  However, there are great ramifications for the gage selection process because it shows that the two large rivers, even after scaling, cannot accurately represent the behavior of a small stream.  For these reasons the Big Creek gage will be used for the remainder of the analyses.

Step 5:  Building confidence in flow estimates.  How good are the data and analysis?

There will be concern regarding the results of the hydrologic analysis using Big Creek due to the shorter length of the period of record.  When faced with this situation, it is beneficial to perform a validation exercise with a system that has a longer period of record.

North Fork Alsea with 31 years in the period of record was chosen for the analysis.  By normalizing the mean annual discharge values for Big Creek and North Fork Alsea and plotting discharge/drainage area versus water year, the user can once again see pattern similarities.  Although, the most useful information this graph displays is that Big Creek data does not occur in a particularly dry or wet cycle.  The line for the mean annual discharge is straddled by the data for each system.  This gives confidence that even with the shorter period of record; the likelihood of overestimation or underestimation is small.

Step 6:  Develop a visual representation of the discharge associated with a typical year

Using the mean monthly discharge/mean annual discharge for the period of record plotted vs. month for a typical water year was compiled.  From the figure, which follows, it is concluded that if diversion will be allowed, it will most likely be allowed during the period of December through March.  April through October all pass 8 % or less of the mean annual discharge, therefore it is this period when diversion could become interrupted by small flows and the need to maintain the 2 cfs instream flow.

Step 7:  Construct a flow duration curve to determine the percent of time water is available/unavailable for diversion.

The 2 cfs minimum instream water rights and 9 cfs adjusted combined water right (should applications be granted) were labeled on the graph of the flow duration curve.  It is noted that approximately 12 % of the time the flow in the river would be too low to provide full supply to the diversion.  According to the figure to follow, minimum instream flow of 2 cfs is likely to be available 100 % of the time for an average year.  However, the demand of 9 cfs will only be available 88 % of the time.  This implies that full supply will be available during a portion of the water year while a reduced supply will be available during other times of the year.  To decide the specific dates when full and partial supplies are likely to be available, a monthly analysis and daily analysis must be conducted.

Step 8:  Perform monthly analysis to search for critical months where flow is insufficient for full capacity.

Monthly averages for the period of record were used to search for critical months.  From the following figures, it is noted that the period of July through October can be considered critical months.  Caution must be taken when allowing for diversion during this period.  Therefore, further analysis on a daily time step must be conducted to determine how often if ever full diversion can take place.

Step 9:  Construct flow duration curves based on mean daily values for each of the critical months to determine what percent of time during each month flow cannot be fully diverted.

The results of the flow duration curves for each of the critical months were tabulated. The amount of time flow is insufficient is equal to 100 minus the exceedence frequency of 9 cfs.  It must be determined if diversion will be permitted at all during months such as September when flow is insufficient 45% of the month. 

Critical month

Amount of time flow is insufficient (%)









Step 10:  Perform flood peak analysis to design protection of intake

For an intake operating on such a small stream, the diverters may choose not to design protection against the 100-year design flood usually used in such projects.  Instead, assume that a flood with a return period of 20 years will be used as a preliminary estimate for the design flow.  Later, during more detailed design, a cost/risk analysis can be made for a range of design flows and return periods.  For now, a Log Pearson type III flood frequency analysis regionalized for the mid coast of Oregon was conducted using the methods outlined in the Analysis Techniques section of this web site was completed.  According to the flood frequency curve, the 20-year flood for North Fork Yachats is 2,000 cfs.  Remember, this value was calculated using instantaneous peak values, making it a brief event and perhaps a conservative estimate.

Step 11:  Summary of analysis and conclusions

The analysis can be summarized as follows:

  • North Fork Yachats watershed is 10 mi2.
  • Big Creek is the gaged system that is used for the analyses.
  • 12% of the time flow in the river is expected to be too low to provide full supply to the city and landowner.
  • Base flow is approximately 5 cfs, 3 cfs above the 2 cfs minimum instream requirement.
  • Full supply cannot be met at all times, however, reduced supply is possible during critical months.
  • The intake must have protection constructed for a design flood of 2,000 cfs.


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