NORTH CASCADE GLACIER CLIMATE PROJECT
Mauri S. Pelto, Director NCGCP
Nichols College, Dudley, MA 01571 Peltoms@nichols.edu

Snowpack Snowpack charts

Ice Worms Ice Worm Charts

Mass Balance Mass balance Data Set

Terminus Behavior Terminus Data Set

Glacier Runoff Glacier runoff  charts

Updated 04/11/08

In the Northern Cascade Mountains, Washington snowpack depth  varies widely because of  local topography and consequent microclimates.   However, the average snowpack within a specific elevation band is determined by regional climate variations.   The graph at right shows the decline in April 1 winter snowpack in the North Cascades since 1946.  This start date is used due to the lack of data prior to 1946.   The decline largely occurred in a step change in 1976-77.  The decline has occurred despite an increase in winter precipitation in the region, as noted by the graph for Diablo and Concrete.  To determine the variation in snowpack accumulation with location glaciers and US Department of Agriculture Snotel sites in the North Cascades are used.  The five key parameters observed are April 1 snow water equivalent, maximum winter snowdepth, the time of maximum snow depth, the timing of the onset of significant snow melt and the rate of snowmelt.  Examining data from Lyman Lake, Thunder Basin, Rainy Pass, and Stevens Pass and the Rainbow, Easton, Lynch and Columbia Glacier we examine the variation with elevation and location within the Northern Cascade Mountains.  The necessity of using Snotel sites and glaciers is emphasized by the difference in mean winter accumulation in snow water equivalent (SWE) which averages 1.17 m at the twelve USDA snotel sites ranging in altitude from 1000-1900 m and averages 2.93m at nine glacier locations ranging from 1650-2200 m.  On July 1 at the nine glacier locations SWE still averages 1.3 m, this is still in excess of the mean maximum at snotel sites.   The change in snowpack during the period of warmer conditions since 1976 is not consistent.  At low elevations sites below 3000 feet, the decline is substantial note Diablo and Concrete below right. At higher elevations above 4000 feet in particular snowpack has not changed significantly from 1976-2008, note below.

The average snow water equivalent observed at six USDA Snotel stations in the North Cascades on April 1 has declined 26% over the last 60 years.  During this same interval winter precipitation has not declined, see below.  Thus, this change in snowpack is due to more winter melting and more frequent winter rain events.

Mean North Cascade snowpack (SWE) on April 1 at Fish Lake, Harts Pass, Lyman Lake, Miners Ridge, Lyman Lake and Stevens Pass since 1976.  This indicates an insignificant increase in April 1 snowpack.

Winter snowfall at Diablo and Concrete since 1976.  This indicates that low elevation snowpack has declined.

MAXIMUM SNOWPACK ACCUMULATION

At the ten Snotel sites from November 1-February 15 snowpack development is rapid reaching 68-80% of the maximum (SWE).  The average maximum SWE for sites above 1500 m is May 5, and for sites below 1500 m is April 10 (Table 2).  The actual maximum accumulation varies with elevation ranging from 0.8 m to 1.6 m, with a mean of 0.98 m for the six sites below 1500 m and a mean of 1.38 m above 1500 m (Figure 2).  The maximum glacier snowpack SWE is distinctly larger with an average accumulation of 2.93 m.

The correlation of daily SWE for the 1989-1997 period at the four Snotel sites between 1500 m and 1900 m ranges from 0.88-0.99.   From 1000 to 1500m the daily SWE correlation coefficient at eight Snotel sites is 0.87-0.99.  The correlation between the high elevation and low elevation Snotel sites is 0.63-0.96.    Each elevation band is then a good predictor of SWE only of the sites in the same elevation band.  The correlation between annual maximum snowpack and total winter precipitation at Diablo Dam is highest for Snotel sites above 1500 m (0.75-0.81). For sites below 1500 m the correlation drops to  (0.63-0.75).   This is expected since a higher proportion of total precipitation falls as snow at the higher sites.

Comparison of annual maximum SWE on glaciers yields cross correlations of 0.82-0.99, indicating the strong regional control of accumulation.  The mean correlationof glaciers to low elevation sites range from 0.37-0.82 for individual glaciers, and for Lyman Lake the best Snotel site from 0.72-0.95.  The two best sites for prediction of glacier snowpack are Lyman Lake and North Klawatti Glacier.

            The mean and maximum SWE depth is variable from site to site, however, the annual pattern of development and relative amount is consistent in response to specific annual climate conditions for each elevation band. 

APRIL 1 SNOWPACK DEPTH     The USDA established a snow depth monitoring program  mid-1930's.  The April 1 date has been a key date for field measurement of snow depth for the duration of the program.  This date falls close to the maximum snow depth at all but the highest altitude sites.  The April 1 SWE has been measured at five sites in the North Cascades since 1946 Fish Lake, Stevens Pass, Rainy Pass, Lyman Lake and Miners Ridge.  The mean April 1 SWE has declined 26% during this interval.  At both Diablo Dam and Concrete winter precipitation has rose 1% and 3% respectively thus, the cause is not less precipitation.  Instead we are seeing more winter rain and melting events.  This is evident in the graph below which compares the ratio of precipitation at Diablo Dam to SWE at the higher altitude stations.  The higher the ratio the greater the percentage of precipitation that is retained as snowpack as of April 1.  Snowmelt or rain are the only two events that can reduce the ratio.  Since most significant melt events accompany rain, this ratio is a good indicator of the increasing amount of rain events at high altitudes during the winter.  We have had wetter winters beginning in 1999,  yet April 1 swe has not increased.  Indicating less of this precipitation entering and remaining as snowpack. The change in mean April 1 snowpack for the 1976-2008 period versus the 1946-1976 period is substantial as noted at right. Since the two period feature nearly identical overall precipitation, the main change is due to temperature.

SNOWPACK ABLATION

Early in the melt season (April-June 15), ablation is dominated by melt at the lower elevation range (>1500m) in alpine basins (Pelto, 1996; Fountain and Tangborn, 1985).   Ablation during May at Snotel sites from 1000-1500 m, averages 0.018 m/day, while at sites from 1500-1900 m average ablation is 0.012m/day, and above 1600 m on glaciers average ablation is 0.08 m/day (Table 3).  Snowpack ablation is reduced somewhat for the Snotel sites are that more protected by surrounding forest (Wells Creek and Thunder Basin). 

Snowpack is lost from the lower sites in May or early June.   The early ablation season, is marked by freezing levels that frequently result in snowfall at Lyman Lake and rainfall at the lower elevation Snotel sites.   Cross correlation of May monthly ablation rates between Snotel sites as a result is poor, as are the daily ablation rates, correlation coefficients ranging from 0.43-0.76 (Table 6).   Correlating ablation at Lyman Lake with precipitation, maximum, minimum and average temperature, indicates the best correlation coefficient for monthly May ablation is with average daily temperature at 0.74.  The early season monthly correlation between the high elevation Snotel sites and glacier sites is modest at 0.72.

Average ablation after June 1 is limited to data from Snotel sites above 1500 m and glaciers.  At the three stations, when snowpack endured throughout all of June, ablation ranged from 0.027 -0.032 m m/day.  June ablation on South Cascade, Easton and Columbia Glacier, during these same June periods, ranged from 0.23-0.29 m/day, averaging 0.027 m/day.  The correlation from glacier to glacier for the same time periods is 0.86-0.99, indicating that ablation conditions are becoming increasingly consistent on glaciers as the summer melt season develops.  Correlation in daily ablation rates for the three Snotel sites is 0.79-0.92   indicating that in the elevation zone from 1500-2000 m across the North Cascades ablation after June 1 has a comparatively low degree of variability.  Correlating ablation at Lyman Lake with precipitation, maximum, minimum and average temperature, indicates the best correlation coefficient for June is with maximum daily temperature at 0.85. 

By early July snowpack beyond the glacier margins is limited, Snotel sites have lost their snowcover, and yet streamflow is still heavily dependent on snow and ice melt from glaciers (Fountain and Tangborn, 1985; Pelto, 1996). From July-September glaciers are the primary area of residual snow and ice ablation.  This region has the highest melt rates during this period, while other inputs are at an annual low (Rasmussen and Tangborn, 1976).  Thus, glaciers ameliorate low flow conditions (Fountain and Tangborn, 1985; Pelto, 1993).   In heavily glaciated basins such a Baker River from 20-45% of the total input is from glacier melt during the latter part of the summer (Pelto, 1996; Post et al; 1971).  This glacier runoff is best determined by direct measurement of ablation on glaciers.  NCGCP  (Pelto, 1996; Pelto and Riedel, 2001) and the USGS (Krimmel, 1998) measurements on glaciers do provide a direct measure of ablation in this elevation band at multiple locations over the last 20 years.   Stakes drilled into the snow and ice of the glacier’s are measured several weeks and or months after emplacement.  This provides the ablation rate.

Ablation measurement on nine North Cascade glaciers for twenty-nine discrete two to six week periods during this part of the ablation season yield mean ablation rates of 0.036 m/day, 0.038 m/day and 0.028 m/day for July, August and September respectively.  The correlation in mid and late season ablation between each glacier exceeds 0.95 (Table 5) indicating the degree to which the regional summer climate is consistent across the North Cascades. 

Comparison of ablation rates and onsite temperature records in the case of the South Cascade Glacier, Easton Glacier, Ice Worm Glacier and Columbia Glacier yield a relationship between air temperature and daily ablation for snow and ice in SWE (Figure 3).   Figure 3 also contains data from Lyman Lake, and Stevens Pass.  The ablation data-temperature relationships is not statistically different for the Snotel and glacier sites.  There is a significant difference between snow and ice ablation for the same temperature

USDA Snotel Sites:  Lyman Lake is a Snotel site at 5900 feet 5 kilometers east of the Cascade Divide.  Winter snowpack depth peaks around May 10.  The mean snowpack depth is 65 inches.  Snowmelt begins in earnest around May 20.  The melt rate until mean snowpack disappearance on July 1 is 4.4 cm/day (1.6 inches per day) (Figure 1 and 2).  The Lyman Glacier monitored by the North Cascade Glacier Climate Project is three kilometers away and at 6400 feet the snowpack remains until  early August the July-August melt rate is 5.6 cm/day.

Stevens Pass is Snotel site at the Pass on Highway 2 at an elevation of 4100 feet.  This site is at the Cascade Crest.  The mean maximum snow depth is 42 inches reached on April 15.  Significant snow melt begins on May 1.  The mean last date of snowpack at the site is June 5.  The melt rate from May 1 to June 5 is 3.0 cm/day (1.2 inches/day) Figure 3.  The lower melt rate than Lyman Lake results from the earlier timing of the onset and conclusion of the melt season.

Thunder Basin is a Snotel site in North Cascades National Park on the west side of the Cascade Crest.  The site is at 4300 feet in elevation.  The site achieves its mean maximum depth on May 1 at 36 inches and begins the melt season on May 10.  The last date of snowpack is June 20.  The 40 day melt season indicates a 2.4 cm/day ablation rate (0.9 inches/day).  This lower rate reflects the location in a deep basin that has good radiational shading and a more forested aspect.

Rainy Pass is a Snotel site at 5000 feet, on the Cascade Crest, but 40 km east of the Skagit Crest which is the primary precipitation divide.  The mean maximum snowpack depth is 48 inches achieved on May 1.   Significant melting begins on May 10.  The melt season last until June 26.  The 46 day melt season indicates a 2.6 cm/day ablation rate (1.05 inches/day).  This is just a little higher than Thunder Basin despite a similar timing and a higher elevation, again indicating the shaded nature of the Thunder Basin site.

Glaciers: At higher elevation than the Snotel sites are the North Cascade glaciers.  Based on crevasse stratigraphy completed each summer and ablation measured during the summer,  Both the residual snowpack for August and the total winter snowfall are determined.  The ablation rate is also observed, though it is for a summer versus a spring period as at most of the Snotel sites.

Rainbow and Easton Glacier are on Mt. Baker, a stratovolcano and the highest summit in the North Cascades at 3285 m. This volcano is host to 11 substantial glaciers with an area of 37.4 km². The Mt. Baker ski area is the site of a snow measurement station, which is located 11 km to the east of Mt. Baker. This snow measurement station has an average annual snowfall of 13.7 m. This station typically loses its snow cover in July.

Accumulation:   The mean accumulation layer thickness at  two different elevations during the 1984-1997 period have been monitored on 8 glaciers throughout the North Cascades (Table 2).  Each of the 8 glaciers is at or west of the Cascade Divide.  The measurement sites were selected in areas where both avalanching, and wind deposition or erosion are at a minimum.  The results indicate consistent snowpack depths west of the divide for the two different elevations, regardless of the year.