When a culvert gets clogged, it has to be cleaned. This is usually done with heavy equipment. At some sites, this may occur dozens of times as beavers easily re-clog the culvert each night. If it is not cleaned every day (at least until all the beavers in the area are killed) the road may be damaged or lost. On a significant thoroughfare, that may represent tens of thousands of dollars for each event.
Beavers are great explorers of the landscape. The thing they seek most is an unoccupied territory with a good place to dam. If you eliminate local beavers, and do not protect the culvert, you have created the perfect destination spot for dispersing beavers. The new beavers will also be shot, and the culvert excavated. “Boom and exhume,” in perpetuity. The acceptance of this cycle, combined with an unwarranted confidence in kill-defense, probably costs tens of millions of dollars each year in North America.