LAKE CLARKE
Current Elevations - Lake depths, collected from 26 cross sections in Lake Clarke
(fig. 3), were adjusted to the normal pool elevation of 227.2 ft above sea level and are
shown on plate 1. The deepest areas of the lake are closest to the dam and range from 30 to
50 ft. The deeper channel close to the left bank, running from just upstream of the dam to
Fishing Creek, is the remnant of the old Susquehanna and Tidewater Canal. The upper
half of the lake is generally shallow; the average lake depth is about 15 ft, with many
islands and sand and coal bars nearWashington Boro. When the lake level is low, much of
the area around Washington Boro is too shallow for boating.
LAKE ALDRED
Current Elevations - Depth-to-bottom data, collected in 1996 from 16 cross sections
in Lake Aldred (fig. 4) and adjusted to the normal pool elevation of 169.75 ft above sea
level, are shown on plate 2. The deepest areas of the lake are not located near the dam but
in spoon-shaped depressions called “deeps” located in the middle and lower parts of the
lake. When Holtwood Dam was built in 1910, a coffer dam, built to divert water away
from construction, exposed six “deeps” along the west bank. Three deeps are shown on
the lake-elevation map and have depths greater than 80 ft; one deep reaches a depth of
about 120 ft, which extends below sea level. How these deeps formed is uncertain but is a
common subject for geology classes and local geological groups. The shallowest area in the lake is the upper part near Weise Island. Unlike Lake Clarke, lake depths decrease
near the dam. Because Holtwood Dam contains no flood gates, sediment is able to
accumulate near the dam rather than exit from the bottom near the flood gates.
CONOWINGO RESERVOIR
Current Elevations - Depth-to-bottom data, collected in 1996 from 23 cross sections
in the Conowingo Reservoir (fig. 5) and adjusted to the normal pool elevation of 108.5 ft
above sea level, are shown on plate 3. The deepest areas of the lake are located near the
dam. Lake depths average about 55 ft along the spillway gates, which are located along
the dam from the east bank to about two-thirds of the way across the river. Lake depths
average about 70 ft near the turbine gates, which are located along the remaining third of
the dam. The spillway and turbine gates are located well below the normal water surface.
Two other deep areas, one across from the Peach Bottom Power Plant and the other below
the confluence with Broad Creek, probably resulted from natural hydraulic scouring
caused by the stream channel shape. The shallowest areas are located in the upper onethird
of the reservoir; lake depths in this area average about 15 ft.
