Originally published in our FALL / WINTER 2002-2003 edition

HEY ... WHERE ARE WE ?    THE MAP'S GOT TO BE WRONG !

They had entered the BWCA at Crab Lake. After two days of some great smallmouth fishing, they decided to venture out on a day trip. During the pre-trip briefing, North Country had told them about a little lake that had some pretty decent pike and largemouth two lakes over from where they were camped.

Portaging away from their base-camp lake took all of about ten minutes. They then traveled about a quarter mile up a small no-current stream to the second portage. They reached the landing, gathered up their fishing gear, paddles, life jackets, and lunch and started up the eighth-mile portage. After a few dozen yards paralleling the creek, the lead trecker called out “That’s the shortest eighth mile I’ve ever walked. They had reached the end of the portage ... right next to a fairly good sized beaver dam.

They figured they would make a few casts before continuing on to their intended destination. They put the canoes in the water and started to make a few casts. Nothing! Eight people worked that pond, and not one fish. Time to push on. Another short portage, and they were ready to lay into the promised largemouth.

By lunch they had caught a few northens, some smallmouth, and a couple of walleye ... but not one largemouth. The same thing after lunch ... no largemouth. And another thing, the map was all screwed-up. The lake was supposed to be figure-8 shaped. This one laid out more like a backwards-7! Frustrated, they headed back to their basecamp.

When they finished their trip, they related most of their tales to us ... including the lousy job we had done routing their wasted day trip for largemouth. And another thing: tell the map maker to correct the map - the portages were marked too long, and the lake didn’t look anything like it showed.

Oh-oh. Something wasn’t right; the maps are never wrong about lake shapes. Then it struck us “the two unusually short portages, beaver dam, separated by non-productive water” ... A new beaver pond had flooded the portage!


While Beavers have developed abilities as lumberjacks, and residential architects, they are best when it comes to constructing water control projects. Damming the flow of moving water is the strongest instinct a beaver seems to have outside of eating and procreating. The sound of running water alone spurs a beaver into action. Beavers will dam anything from a spring to a fast-running stream. They are unable to harness the largest rivers, but are very creative in using stream boulders and islands to anchor dams that span larger and faster flowing rivers than one might think possible.

Rivers and streams have fluctuating water levels and also have a limited amount of safe foraging habitat. Dams built on these water bodies raise the water level so that streamside vegetation is flooded and food is accessible without leaving the water. Stabilization of the water level allows a lodge to be built along the river bank where it would otherwise be washed away.

While many people have seen beaver dams, few have observed beavers constructing them. Beavers do most of their work at night and only dedicated researchers, trappers, and amateur naturalists (versus canoe trippers)have the patience to sit up after dark to watch Nature’s engineers at work.

One of the first activities of a colony of beavers moving into a new area is to construct a dam. Most dam building occurs in the early spring (May here in the canoe country), and late summer, with a couple of weeks of intense activity in each season.

While human engineers select sites for their dams so that costs are low, dams stability is high, and results are predictable, beavers seem to have a more haphazard approach. Some beaver dams appear to be in an ideal location, while others are located in an area where they are difficult to construct and don’t provide maximum benefit. In these cases, there is often a far better location near by. It is believed that beavers learn through experience and that older beavers make better selections than younger animals.

Beavers respond to both the sound and the feel of running water and often choose locations for dams that are close to places where the water is running fastest and making the most noise. On a number of streams we have personally traveled, we have found dams in places where the river had narrowed and was merely running noisily over exposed rocks.

Adults initiate most dams, but are aided in construction by all but the youngest colony members. Beavers add sticks to the dam by pushing them into the existing structure using their powerful jaw and neck muscles. The result is a strong latticework that is very difficult to pull apart. Once a dam’s crest is above water level, mud is added to restrict flow through the network of sticks. The mud is usually daubed on the upstream side of the dam so that the water forces the mud into the inner crevices. Eventually the leaks are filled and the dam holds back almost all of the water.
As water backs up behind the dam, it floods a large area. Typically, the water finds another channel and flows past the dam to the side. In these cases, the beavers will build additional small dams to capture that water flow and to increase the efficiency of their main structure.

There is no register of the largest and longest beaver dams, but local trappers who wander the woods and hike back into the small impoundments to set traps in winter bring back tales of some monsters. On our own travels, we encountered a dam just off of Basswood Lake that was almost eight feet high on the down stream side. In Quetico Park, we portaged around a dam that spanned 100 yards from shore to shore forming one of those “not-on-the-map” ponds. Years ago, while traveling The S-Chain, again in Quetico, we crested a rise on a portage to find not a creek crossing as we expected, but a pond several hundred yards across. Sure enough, the portage continued on the other side of the pond. That pond is still there, and is regularly pointed out during mapping sessions involving that segment of lakes.

The action of paddlers crossing over a dam, flood events, and the continued force of water all eat away at the dam. Beavers regularly conduct inspections of their work to patch leaks and shore up weak spots. Small holes are repaired by bringing up mud from the bottom and by pulling materials from the sides of breaks. Medium-sized breaks require more effort. Adults often swim into the hole to block the flow of the water and pull branches into the hole from the side of the break. They then back out of the hole and push mud into the hole to seal it. Beaver dams can last for decades with the descendants of the original builders caring for their ancestral structures.

As for our party that doubted our accuracy with the pre-trip mapping ... give us just a little room to maneuver here. Beavers can dam a narrows in a few days, and if the terrain is right, can flood a several acre dip in the topography within weeks. We think we are pretty good with the map markings, but even we can’t keep up on every beaver within 2,300,000 acres.

Besides, if someone really has to know what is around every bend, they might try looking into the Jungle River Boat ride at Disney World: listen carefully to the tour guide –- just before he shoots the attacking hippo.

The Boundary Waters and Quetico Park form the only lakeland wilderness in the world.
(You know ... the one set aside for the big kids.)