Beavers and the Risk of Flooding in Chebacco Lake and Alewife Brook

Welcome! This Wiki has been created to post activity pertaining to the maintenance of water flow from Chebacco Lake and through the Alewife Brook outlet. The area in question is in the towns of Essex and Hamilton, Massachusetts.

This Web site is maintained by the Chebacco Lake & Watershed Association (CLA). For further information, contact Dave Lash at 978-468-6633 or by email at "dave at"

For newcomers: About Chebacco Lake, our ongoing beaver problem, and alewife passage

4/11/07 by Dave Lash:
Monday night's meeting with the Essex Board of Selectmen was well attended by over two dozen concerned property owners. Also attending were organizer and State Rep. Brad Hill, Senator Bruce Tarr, State Rep. Tony Verga, Brad Hill from Marine Fisheries, Kathy Leahy from Mass Audubon, David Rimmer from Essex County Greenbelt, and members of Essex boards and commissions.

Our goals for the meeting were 1) to explain the increased threat to flooding in Chebacco Lake and Alewife Brook as a result of the increased beaver population throughout the watershed (exacerbating the lack of cleaning and dredging in the brook), and 2) to request the formation of a study task force to recommend a regional management solution.

The PowerPoint slides presented at the meeting can be viewed by clicking here.

2/1/07 by Dave Lash:
The trapper returned in early January; however, three days after he finished there was damming again. The Board decided not to try a 3rd time (ice coming on; spring just 8 weeks away). Sue and I cleaned the fence thoroughly. The Dawn Patrol (Slats, Sue, and Nancy) are doing inspections twice a week. There is damming near the fence, but so far, the fence is not being obstructed and water is flowing quite well.

Special meeting has been scheduled for Feb 15.

Water level continues to drop; lake is frozen so accurate reading is difficult: approximately 46' 6".

12/29/06 by Dave Lash:
I returned this afternoon to inspect as planned. Some re-damming had occurred since Wednesday, unfortunately. About 5 of the openings nearest the shore had been closed; the fence area was untouched. I called the trapper and he will let me know next week when he can return; he will charge us a daily rate at a discount given the situation. We have adequate funds in the account. I'm optimistic that this is a minor setback.

In the last two days, the breaching has helped drop the water level about 2" to approximately 46' 8".

12/27/06 by Dave Lash: Here's where we are on the water level front:
1. The trapper finished his work about 2 weeks ago. I will be forwarding his invoice to Betty for payment.
2. I finally got down to the dam this morning (12/27) to inspect and do some breaching.
3. There was a 7 or 8 inch head and water was trickling over the dam along the length. No breaches were visible.
4. The beavers had collected a huge supply of food branches around the lodge.
5. I opened about 10 gates and got quite a bit of water flowing.
6. I did what I could at the fence, but a complete cleaning is required.
7. I will inspect again on Friday to see if any re-damming has occurred (heaven forbid).

Barring more precipitation, I would expect the lake level to drop 6 to 8 inches over the next 2 weeks. Here are the lake elevation figures for reference (figures approximate):

- height of current dam, aka current water level: 46' 10"
- height of original dam in 2003: 46' 7"
- height of second downstream dam (out of our control for now): 46' 3"
- optimal lake elevation (my personal opinion): 46' 0"

Therefore, in my view, the current water level is approximately 10" above optimum, but hopefully will drop 6 to 8 inches after the trapping and breaching.

5/4/06 by Dave Lash: beautiful day hitting 70! Much nicer for inspecting the fence. My report:

The fence
  • water flowing freely through the fence.
  • a few sticks stuck through the fence at the pinchpoint indicating some beaver activity but no blockage and they clearly did not get inside.
  • some minor mud and sticks upstream of the pinchpoint below the water mark -- as before
  • foot of the fence (the downstream end) -- I removed debris for 20 minutes...might be beavers' work with it, but much is just sticks and weeds in the water getting hung up.
  • I'll inspect again this weekend.

Water level
  • because of the rain the last two days (1.5" Boston data), the lake is up about 6" (currently 46' 4").
  • there is a natural 4-5" head at the fence, indicating that the rainfall is being held back at the dam...we know from past observations that the lake will drop about 1-2" per day after an average rainfall like this when the full 8' channel is open...we'll see what happens with the 3-4' opening we have now at the fence.
  • I breached 3 other gates just to help the flow.
  • in any event, the current water level is not a flooding concern.

  • none spotted at the fence
  • at Apple Street bridge, 36 alewife passed between 12:55 and 1:10 p.m. Water level: approx. 3" over the weir. Water temp: 15 degrees C.

5/3/06 Mike Callahan's email reply:
Hello Dave,
Thanks for the wiki update. Very impressive!
So strange that the beavers got inside to dam the outlet.
From the photos the fence doesn't appear to be damaged providing an opening for the beavers.
If I didn't see the photo of the dam inside the fence I wouldn't have believed it!
Seeing the area yourself do you have any ideas how they got in?
Out of hundreds of these fences I have only seen damming inside this type of fence once.
It was last Spring in Charlton, MA. A small beaver who had a history of damming a culvert before the fence was installed was able to fit through the fence with sticks and mud and dam the culvert inlet. Fortunately by June it had grown larger unable to fit through so the damming stopped (hopefully forever). My suspicion is that the same thing might be happening here since these beavers were so accustomed to rebuilding their dam.
Any thoughts or further updates would be appreciated.
Beaver Solutions

5/2/06 at 3:30 p.m. by Dave Lash: it's been raining since last night; the lake level (approximately 46' 2") is up 5-6" since last week. At the Apple Street bridge, the water level is 1-2" over the weir, sufficient for alewife passage in the cutout section.

Pond Street dams: evidence of some damming. A large log has been dragged across the upper dam and alewife passage is extremely tenuous and possibly completely blocked.

Outlet fence: to my horror, I found the beavers had figured out how to get inside the fence and had completely dammed across the pinchpoint of the fence. A head of 15-16" was found; absolutely no alewife passage was possible. Furthermore, there was the downstream face of the fence was stuffed with small debris and restricting flow. It took two hours, but I opened it up and restored the flow of water. Hopefully, this cleanout and the rain will relieve the low water Peter observed.


Another email from Peter: I think the low water started last Wednesday. I dont think fish can even get through the riffles upstream of the landfill access road except at high tide...let alone get up to the beaver dam. Is the dam serving a positive role for Chebbaco Lake in keeping the water level up during this dry spring?

5/1/06 by Dave Lash: I received the following email from Peter Phippen: Dave- The brook went really low...all of a sudden, last Wednesday or Thursday. Were the beavers successful at finally blocking off the channel? -Peter

4/15/06 by Dave Lash: a Lake Association work party burned brush (under permit) most of the day. It took 3 large fires to consume the brush the beavers placed and the humans removed over just that last year. Thanks to volunteers Chuck Bencal, Sue McLaughlin, Bill Mahoney, and Nancy Doyle.

We cleaned the fence thoroughly, noting that the beavers had begun damming along the side of the fence and flow was becoming a bit restricted. There was no sign of rebuilding of the second dam at the outlet or of the two Pond Street dams.

Alewife: from 10 a.m. until noon, alewife ran continuously through the fence --- absolutely fabulous to see. We estimate that several hundred came through in this time. Since this was our first observation of the alewife reacting to the fence, I will record what we saw. The alewife seemed to travel in small schools of one to two dozen. Upon confronting the fence, there was definitely some hesitation and caution. After hovering and sizing up the situation for a minute or less, they entered the fence. The current had scoured a depression in the streambed below the floor fencing. The alewife found safety in this depression, moving cautiously upstream until they had passed the pinchpoint of the fence. When the fence widened (and the depression disappeared), the alewife darted through the final 10 feet and clear of the fence. We saw this pattern over and over and concluded that the fence design does not, in fact, pose any meaningful deterent to the alewife.

4/12/06 and 4/14/06 by Dave Lash: inspected the two lower dams and the fence: no sign of new beaver activity.

4/11/06 by Dave Lash: Brad Chase and a colleague traversed the Alewife Brook from the Pond Street bridge all the way to the fence and beaver dam at the lake outlet. They breached 5 dams in all and cleaned debris out of the fence. Brad felt alewife might have been able to sneak through the dams although significant deterents existed.

4/4/06 by Dave Lash: The beginning of our spring saga: After a winter of bliss (the beavers poked at the fence but never blocked it), I arrived on this date and found the beavers had built a new dam about 30 feet downstream. I did not have hip boots that day and therefore could not breach.

Given that this is the month that the alewife beginning traveling up the brook into Chebacco Lake to spawn, I stopped and asked downstream if any beaver activity had been noted. I was told two new dams had been constructed recently just upstream of the Pond Street bridge. Upon inspection, I found one of the dams and noted it had a full 12" head. Worried about the passage of the alewife, I called Peter Phippen at Eight Towns and the Bay. Peter noted that the water was still cold and the alewife run probably hadn't started in earnest as yet; he recommended contacting Brad Chase at Marine Fisheries.

On or around this date, I saw a school of approximately 100 alewife swimming in the lake. The sun was shining brightly and I could have made an exact count if I'd wanted to.

It should be recorded that the lake water level this spring is probably the lowest in 20 years or more.

10/23/05 by Dave Lash and Jack Akin: Absolutely no beaver activity seen on the upstream end...hurray!...they sealed the dam into the sides of the fence with sticks and mud as expected...the downstream end definitely got their attention...see below.

Here's what the fence looked like today:

It's hard to see much at this size...check out these 3 enlarged photos:
Upstream trapezoid end
Mid section at dam
Downstream end

At the downstream end, the beavers had threaded 15 to 20 branches through the fencing, but were clearly not sure what more to reduction of flow due to these branches...leaves were catching in the fencing...we cleared all branches and most of the leaves. We'll have to keep monitoring to see what the beavers do next.

See close-up photo of downstream end of fence where beavers were working.

10/22/05 by Dave Lash: Water flow was brisk and unimpeded. The beavers have definitely taken notice. They have packed the ends of the dam where they join the fencing. The upstream, trapezoidal portion of the fence showed no evidence of beaver activity at all; however, the beaver are testing the downstream, rectangular portion -- wedging some long branches through the fencing.

10/18/05: Mike Callahan installs the fence assisted by CLA volunteers Dave Kerr and David Lenzi.

This Web site is maintained by the Chebacco Lake & Watershed Association (CLA). For further information, contact Dave Lash at 978-468-6633 or by email at "dave at"