Physical and Chemical Data
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Lake level has exhibited
considerable long-term variation since the late 1890's. From
1910 to 1942 the lake dropped about 4 m in elevation, but
returned to the 1910 level by the late 1950's. The lake
fluctuated by 1 m above and below a bench mark level of 1882 m
observed from 1958 to 1985. Between 1986 and 1994, the level
dropped to about 3.5 m below the long-term average elevation.
Since then the lake has risen to nearly the bench mark level.
The surface of Crater Lake seldom freezes over,
owing to the heat content of the massive lake volume and wind
mixing. The only known occurrences of ice and snow cover
occurred in 1948 and 1985. In 1986 the lake was nearly covered
by ice at various times in January and March.
In winter and spring the water mass in Crater
Lake circulates to a depth of between 200 and 250 m by wind
action and cooling. The deep lake is mixed in winter and early
spring each year when relatively cold water near the surface
sinks and exchanges positions with water in the deep basins of
the lake. In late spring or early summer the temperature of the
lake near the surface increases and a thermocline forms between
July and September. The depth of the epilimnion is between 5 m
and 20 m and is usually deepest in fall because of cooler air
temperatures and an increase in mixing generated by storms.
Maximum near-surface temperatures typically ranged from 14 to 19
degrees C from June or early July to mid-September. The
metalimnion extends to a depth of about 100 m; thus, most of the
water volume is a cold hypolimnion. Annually water temperatures
do not vary by more than 1 degrees C below a depth of 80 m. The
temperature at the bottom of the water column is about 3.5
degrees C.
The average pH of the entire water column of
Crater lake is 7.5. Average near-surface pH ranges from about
7.6 to 7.7, whereas the pH decreases with increased lake depth
and is about 7.3 at a depth of 550 m. The average total
alkalinity and specific conductance are 27 mg/l and 115 mhos/cm,
respectively. Both variables increase slightly with increasing
lake depth. The average concentration of dissolved oxygen is
relatively uniform in the water column in spring, but decreases
to about 90% saturation at 550 m during the period of thermal
stratification. Nitrate-nitrogen, ammonia-nitrogen, Kjeldahl-nitrogen,
total phosphorus and orthophosphate-phosphorus occur in low
concentrations. Nitrate-nitrogen is virtually undetectable in
the upper 200 m of the water column; however, the concentration
increases to maximum with increased depth. Kjeldahl-nitrogen and
ammonia-nitrogen decrease in concentration with increased lake
depth, whereas orthophosphate-phosphorus increases. Total
phosphorus is nearly uniform in concentration throughout the
water column.
Secchi disk clarity readings are generally in
the mid-high 20s and low 30s each year from June through
September. The highest (deepest) readings usually occurred in
late June and July and the lowest in August. Nonetheless, there
were some differences in the temporal patterns during the 17
years (Figure
4). For example, in 1997 the readings from late July
through August were the deepest recorded during the study
period, whereas Secchi disk readings in late July 1995 were the
shallowest readings from 1982-1999.
