92 
BULLETIN OF THE BUREAU OF FISHERIES 
the head of navigation at Minnehaha Falls, near St. Paul, Minn. It is a little 
above the halfway point between Lake Itaska and the Gulf of Mexico (a distance 
of 2,553 miles). It is above the greater tributaries — the Missouri (200 miles) and 
the Ohio (375 miles). In fact, no regularly navigable streams or waters enter the 
Mississippi above Keokuk except the Rock River and Hennepin Canal, connecting 
with the river just below Rock Island, 111., and the St. Croix in Minnesota. The 
tributary streams of most direct possible effect upon the dam are the Des Moines, 
about 3 miles below the dam; the Skunk River, entering the lake 35 miles above the 
dam; the Iowa River, 72 miles above the dam; and the Rock River, 119 miles above. 
The site of the dam is just at the foot of the old Des Moines Rapids of the Mis- 
sissippi, where the valley is comparatively narrow. (Figs. 1 and 2.) “The river at 
this place, in its natural condition, is about 2,600 feet in width at ordinary low water 
and about 5,500 feet in width at flood stages.” (Clark, 1911, p. 203.) The dam, 
with its abutments, is nearly 1 mile long, and the fall is 32 feet, more or less, according 
to the stage of river level above and below. Low water just above the dam, which 
was formerly about 485 feet, is now 519, referring to the Memphis datum. High 
water was formerly 505 and is now 525. The pool, or Lake Keokuk, extends to a 
point a little above Burlington, or about 40 miles above Keokuk. The expanse of 
the lake is approximately 60 square miles — actually 58 square miles at low water, 
as compared with 36 square miles of river surface in former times at low water, and 
64 square miles at high water as compared with 54 square miles in former times at 
a corresponding volume flow of the river. 7 
Returning to the dam, we find, beginning from the Iowa shore, the Government 
shipyard and dry dock, the lock (110 by 400 feet), and a short section of dam con- 
necting the upper end of the lock with the lower end of the power house, which 
stands in the river 700 or 800 feet from the Iowa shore. Although the lower half 
of the power house is unfinished, except for its foundation (an integral part of the 
dam), the entire structure is one-third of a mile long. It extends upstream and 
nearly parallel with the Iowa shore to connect with the west end of the dam proper, 
which in turn extends across the river a distance of four-fifths of a mile to the 
Illinois shore. 
We will now examine each of the principal structures constituting the dam (the 
dam structure, the power house, and the lock), as regards the general relation of 
each to the movements of fish. Additional details will be found in the companion 
report, in which the several species of fish are considered separately. 
THE DAM PROPER 
The dam structure is composed of 119 spans, each consisting of two piers sup- 
porting an arch, while the arches uphold a causeway. Between the piers are spill- 
ways, over which the water flows. Each of these spans measures 36 feet, center to 
center, the spillways being 30 feet wide and the piers 6 feet. The height of the 
spillways is 32 feet; their upstream face is vertical and the downstream face an 
ogee or compound curve, delivering the water in a horizontal direction down the 
river. For the protection of the base of the dam from scouring, a broad, low, con- 
crete apron has been constructed. 
Between the top of each spillway and the under side of the overhanging cause- 
way is an arched opening about 19 by 30 feet (5.1 by 9.1 meters), which will permit 
7 Information supplied by the Mississippi River Power Co. by letter datod May 19, 1927; high water corresponding to a flow 
of 300,000 second-feet and low water to a flow of 50,000 second-feet. 
