The Seasonal and Annual Flows of the Dungeness River

by Welden and Virginia Clark

The data for western rivers are typically recorded by “water year.”  Water year 2001, for example, began in the prior calendar year on 1 October 2000, and ended after twelve months on 30 September 2001. The rationale for this model is because western weather patterns typically exhibit a stormy wet season in the six months from October to March, then a less dynamic dry season in the six months from April to September.

For the Dungeness and similar rivers fed by mountain snows, this pattern is especially pronounced. The period October to December typically has rainstorms and snow, whereas the period January to March has snowpack accumulation with fewer storms.   April initiates the spring and summer snowmelt period, beginning with a sustained increase of runoff, a broad peak through May and June, and then a gradual decline through July.  The water year concludes with the lowest river flows of the year in late summer and early fall.   Figure 1 illustrates the observed bimodal pattern for Dungeness flows averaged over half-month periods. The minimums, the means, and maximums shown for consecutive half-month periods emphasize the greater variability and skewness in the wet season.

The long-term pattern for Dungeness flows:

We are fortunate that river flow data for the Dungeness River are available for over 73 water years.  These data were collected for 2+ years at the beginning of the 1900s at a gauge near the present Schoolhouse Bridge, then 7 years, 1924-1930, at river mile 11.5, and then continuously at the same location since 1938.  The water-year mean flow since the 1920s has been about 380 cfs (cubic feet per second), equivalent to 275,000 acre ft.   The lowest water year on record was 1977 with a mean flow of 197 cfs and the highest water year was 1999 with a mean flow of 697 cfs. 

The overall mean values are clearly misleading, as the flows vary widely from year to year due to typical variations in precipitation and winter snowpack (Figure 2).  The river-flow pattern is punctuated by recurrent climate fluctuations.  The ENSO years (El Nino/Southern Oscillation) happen irregularly at about 3 to 7 year recurrences, with cooler, wetter periods in the northwest during La Nina and warmer, dryer periods during El Nino.  Our analyses show lower precipitation and snowpack here during El Nino years; larger studies of  NW basins show similar results.

University of Washington oceanographers have recently identified another natural cycle in sea surface temperatures of the north Pacific Ocean called the Pacific Decadal Oscillation (PDO).  The PDO seems to show a shift between warmer/dryer or cooler/wetter years about every 20 to 30 years in the Pacific Northwest.   The Dungeness watershed tends towards more rain and snowpack, and thus increased river flows, in the cooler/wetter PDO phase.    The approximate years of phase changes are shown in Figure 3, occurring about 1925 (cool to warm), about 1947 (warm to cool), in 1977 (cool to warm), and probably about 1998 (warm to cool).   These correspond to discernable shifts in precipitation, snowpack, and river flows in the multi-decade intervals.

Longer-term global climate change:

Additionally (and much in the news these days), long-term climate change such as global warming is projected to occur over the next century.  The major consequence for the Pacific Northwest is warmer winters with more rain-on-snow storms, which will probably cause more winter flooding but reduced

spring and summer snowpack runoff.   The likely result for our rivers will be lower water-year mean flows and substantially reduced late summer and fall instream flows.  

Interestingly, a similar warming occurred from the late 1800s to 1923, providing a chance to analyze just such a long-term climate change (see the gray dots and line of estimated river flows in Figure 3).  This is a conjectural model of river flows based on the slowly warming and drying climate following the ending of the ‘little ice age,’ which is considered to have lasted from about 1450 to near 1890.  Our conjecture suggests a typical annual Dungeness flow of about 500 cfs at the turn of the century, consistent with the several years’ gauge data obtained then, and significantly higher than at present. Observations of the retreat and disappearance of glaciers of the Olympics in the early to mid 1900s also show evidence of such warming.

Impacts of low instream flow in the Dungeness:

The most serious consequence of low flows in the Dungeness during late summer and fall is inadequate water for salmon.   This problem is clearly exacerbated by the diversion of water from the river for agriculture.   Such diversions began in 1896 with the construction of the Sequim Prairie ditch, which crosses under the ‘bump’ in present Priest Road east of Railroad Bridge Park.   The creation of nine irrigation ditches by the 1920s led to significant summer/fall diversions and substantially reduced downstream river flows.   Recent changes in water management and conservation, such as close monitoring at diversion outtakes, have helped to decrease these water diversions to less than 70 cfs.   Recent changes in land-use and decreases in irrigated cropland have also helped to decrease water diversions.  Current watershed planning is continuing with the objective of establishing an instream flow rule and target flows, providing a ‘water right for salmon habitat’ based on WA State and Federal Endangered Species Act guidelines.  

Flood flow events

Flood events on the Dungeness typically occur when heavy rains from warm, wet ‘pineapple express’ rainstorms fall on snow pack in higher elevations, causing high rates of snow melt in addition to the rainfall.  In an analysis we did in 1995, then updated in 1998, we found 125 events in 67 years that had one or more consecutive days of daily mean flow at least 1500 cfs, which is high enough to cause bedload movement and channel erosion.  Over 80% of these events occurred during the Oct-March wet season.   Of these, 17 reached 5,000 cfs or higher and 9 flooded above 6,000 cfs.  Since 1924 only two flood events have exceeded 7,000 cfs:  7120 cfs in November 1990, and the current record, the recent record flood of 7610 cfs on 7 January 2002.

Hydrographs of river flows

Water-year hydrographs of daily mean flows provide a useful comparison between years.  As examples, Figure 4 illustrates the third lowest water-year, 208 cfs mean flow in 2001 (197 cfs in 1977 was the lowest).  For comparison, Figure 5 shows the record high water-year in 1999 of 697 cfs.  Hydrographs of intraday flows are also useful for analyzing flood events.  Figure 6 shows the hydrograph over the days of the recent flood event that peaked at 7610 cfs late on 7 January 2002.

[NOTE: much of the material in this article is adapted from Chapter 5 of  “Keys to an understanding of the natural history of the Dungeness River System”, (1996, Welden & Virginia Clark), and later updates.   Much of the material, together with reference citations, is online in the ‘Exhibits’ section of the River Center website at <www.DungenessRiverCenter.org>