This image shows a typical beaver dam structure, a once-common sight throughout North America until the beaver was hunted to near extinction in much of its normal range.
The description states:
CANADIAN WILDLIFE POST CARD SERIES
Beavers build dams for their won protection, and in turn conserve and store water, benefitting many forms of wildlife.
— There are both benefits and disadvantages to these dams. The risk of dam failure and flooding are among the major problems they pose when near populated areas.
This image likely dates from the 1950s.