In building such a dam as this the difficulties to be contended against
were unusually great. It was required to make it as near perfectly tight
as possible and be, of course, always submerged. Allowing for water used
by canal and slide and the leakage there should be a depth on the crest
of the dam in low water of 2.50 feet and in high of about 10 feet.
These depths turned out ultimately to be correct. The river reaches
its highest about the middle of May, and its lowest in September. It
generally begins to rise again in November. Nothing could be done except
during the short low water season, and some years nothing at all. Even
at the most favorable time the amount of water to be controlled was
large. Then the depth at the site varied in depth from 2 to 14 feet, and
at one place was as much as 23 feet. The current was at the rate of from
10 to 12 miles an hour. Therefore, failures, losses, etc., could not be
avoided, and a great deal had to be learned as the work progressed. I
am not aware that a dam of the kind was ever built, or attempted to be
built across a river having such a large flow as the Ottawa.

The method of construction was as follows. Temporary structures of
various kinds suited to position, time, etc., were first placed
immediately above the site of the dam to break the current. This was
done in sections and the permanent dam proceeded with under that
protection.

In shallow water timber sills 36 feet long and 12 inches by 12 inches
were bolted to the lock up and down stream, having their tops a uniform
height, namely, 9.30 feet below the top of dam when finished. These
sills were, where the rock was high enough, scribed immediately to it,
but if not, they were 'made up' by other timbers scribed to the rock, as
shown by Figs 4 and 5. They were generally placed in pairs about 6 feet
apart, and each alternate space left open for the passage of water, to