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A History of Lake Levels
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In 1886, a group of people representing the U.S Geologic Survey,
measured the depth of Crater Lake in several places. Using piano wire
and a lead weight, they determined that the greatest depth of Crater
Lake was 1996 ft. In 1959, a similar survey from the U.S.G.S. corrected
the earlier measurement using sonar and established the depth at 1932
ft. This depth is referenced against a surface elevation of 6,176 ft.
Because the Lake loses water daily by evaporation and seepage, there
are times when the Lake depth must be less than the 1959 value.
Likewise, since Lake inputs vary from year to year, there are years when
the 1932 ft. depth is exceeded.
Crater Lake is unique for many reasons including its isolation from
surrounding streams and rivers. The primary input for the Lake is the
annual precipitation the region receives which is close to 69 inches for
the long term average as measured at Park Headquarters. The Lake level
rises annually from October to April since input exceeds output. As
precipitation lessens, the level of the Lake stabilizes until late June.
This condition can be explained on the basis of a balance between
evaporation and seepage momentarily matching reduced precipitation and
run off from melting snow. For the rest of the summer the Lake level
falls at a daily average rate of .675 cm/day or about 25 hundredths of
an inch per day. The Lake level is normally at its lowest yearly amount
at the end of September. For the Lake level to return to the same value
year after year, the input as measured at Park Headquarter must be 66.9
inches which is close to the long term average measured at this site
(Redmond, 1990).
Lake level measurements have been recorded inconsistently since the
early 1880's. These values have been adjusted to a September 30 date
for comparison. The lowest Lake level was recorded on September 10,
1942 when the Lake dropped to a surface level of 6,163.20 ft. This
historic reading is related to low precipitation amounts observed
regionally during the 1930s. In 1975, the Lake level reached an
historic high when it rose to a level of 6,179.34 ft. There is some
evidence that the Lake never gets much higher than this 1975
measurement. Lichen stains on rocks near shoreline indicate that the
water may have not been above 6180.50 ft. This evidence is also
consistent with trees both dead and alive rooted just a few feet above
the observed Lake level maximum.
Since Crater Lake has no other significant input source, the Lake
level is subject to abrupt changes year to year when snowfall amounts
vary. For example, the Lake level rose 2.5 ft between 1951 and 1952.
Later it fell 3.40 ft between 1976 and 1977. With more normal amounts
of snowfall, little change occurs to the gross Lake level from year to
year. As noted earlier, the range of Lake level measurements has varied
some 16.14 ft during the last century. The 30 year average for the Lake
level is 6,175 ft. This is slightly higher that the average for the
period 1907 to 1988 which is 6,170 ft which obviously reflects the low
water level observed in the 1930s and early 1940s.
During the summer of 1991, the Lake level was estimated at 6,170.80
ft on July 31. Assuming no significant precipitation through September,
by September 31, the Lake level may be near 6,170 ft, the lowest level
since the early 1950's. As with the low Lake levels earlier this
century, the present trend appears related to less than normal
precipitation amounts beginning in the mid 80s. A hundred years of Lake
measurements have taught us that we should not assume that the level of
Crater Lake does not change. 30 year or even 100 year averages can be
very misleading. What we observe one year or one decade is no
indication of what can happen the next. Furthermore, since Lake level
changes are related to regional climatic events, it is impossible to
forecast the next season's Lake level accurately. When we tell the
public that Crater Lake is 1932 ft deep, we are relating a single
measurement observed at a given time during the recent past. As with
any dynamic system, the depth of Crater Lake and its elevation above sea
level is never a fixed pair of values since each rise and fall according
to forces imposed by nature from outside the caldera.
Redmond,K.T. 1988 Crater Lake, An Ecosystem Study.
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