2011 - Cooperative Institute for Research in Environmental Sciences ...

William Lewis, Jr.
Trouble at Grand Lake
Grand Lake is the
largest and deepest natural
lake in Colorado. The
name and first-place rank
of this lake would suggest
that it is gigantic, but
actually it is rather small
(507 acres in area and 265
feet deep), reflective of
Colorado’s population
of abundant but miniature
glacier lakes. The
setting for Grand Lake
is the Colorado River
headwaters (8,367 feet)
near Rocky Mountain
National Park, which for
a century has attracted
visitors and cabin builders
who want to be near
the lake.
Residents and admirers of Grand Lake are upset by
what they perceive to be declining transparency and loss
of the deep-blue color of Grand Lake. They attribute these
changes to water management under the supervision of
the Northern Colorado Water Conservancy District’s Colorado-Big
Thompson Project (C-BT). The District passes
mountain runoff into two storage reservoirs, Granby and
Grand Lake in summer, looking west.
Shadow Mountain, which in turn pass water to Grand
Lake. Water exits Grand Lake to the east, through the Adams
Tunnel, which was completed in 1947. Since 1947, the
passage of water through the tunnel to the agricultural
lands east of the Continental Divide has been augmented
in volume and diversified in source to the extent that the
lake now is dominated by imported water rather than
water from its own watershed.
The exact characteristics of Grand Lake prior to any
water development are unknown, but there is a tantalizing
record of transparency measured by Secchi disk (a
white disk lowered to the point of disappearance in the
water column) recorded as 9.2 m in 1941 by Professor
Robert Pennak of the University of Colorado faculty. At
present, the Secchi depths observed in the lake average
3.5 m. Friends of Grand Lake suspect that water management
has greatly impaired the transparency of the lake.
With support from a collaboration involving Grand
County and the Northern District, the CU Limnology
Center, under leadership of James McCutchan, analyzed
data relevant to optical characteristics of the lake and its
water sources. The data indicate that color from dissolved
substances, particles and nutrients that generate growth
of algae in the lake’s water column all account for some
extinction of light in the lake. A comparison with the
waters draining from the watershed (which are very low
in nutrients, particles and dissolved organic matter) suggests
that the original transparency of the lake was much
higher than it is today because of the use of the lake as a
passageway for water from other sources. The Northern
District is probing the validity of the analysis, while it
also considers ways of moderating the effect of water
transfers on water quality.
Transparency maps of Grand Lake shown as Secchi depth (depth at which a standardized white disk can be seen). Left: With water
transfers in progress. Right: With water transfers experimentally shut down for four days. Data from GCWIN monitoring.
CIRES Annual Report 2011 39