Fish Ladders Background Information

The history of dams is a long and oftentimes controversial one. In fact, the oldest known stone dam in the world, the

Sadd Al-Kafara in Egypt, was constructed some 4,000 years ago for reasons that are not quite clear and disputed by

historians to this very day. Dams are built to created reservoirs for drinking and irrigation and to harness the power of

the water itself. It is understood that these structures change the ecology and the hydrology on both sides of the dam,

but it is hoped that the decision is not taken lightly and that the benefits will far outweigh the costs.

The earliest known fish passages comes from scant writings of local people helping fish to pass obstructions in rivers in

France in the 17th century. It is believed that branches were bundled together and placed in such a way as to allow the

fish to pass over whatever was blocking their way. It would be two hundred years before a design was patented for this

very purpose, but the importance of allowing fish to migrate has been on people’s minds for centuries and even has

been the cause of violent disputes. Historic records show that Native Americans protested the building of a dam on the

Presumpscot River in Massachusetts in 1739, winning an injunction against the dam builder since it would deprive them

of a seasonal fish catch. Two years later at a site north of the contested dam, Native American’s burned down a mill that

they had earlier protested against. Data of such conflicts on the Bronx River have not been brought to light if they exist.

For a great lesson on the debates that have taken place over such issues, visit the Smithsonian webpage listed in the

Resource section.

An important question that arises today is what to do with the dam once it is there and no longer in use. Should it be

removed? Should it be left in place? What are the costs and benefits of removing the dam or leaving in it place? There

are strong argument in favor of removal (http://internationalrivers.org/en/node/571 ) and arguments against removal

(http://familyfarmalliance.clubwizard.com/IMUpload/Dam%20Removal%20Econ%20Studies.pdf ) which can spark quite

a lively debate. At present on the Bronx River, the plans are for the construction of fish ladders.

Activity

“The Ladder to Success”

After having participated in the Alewife lesson, students have learned that the dams on the Bronx River are what may

have had the greatest effect on ending the alewife migration. In this lesson, students will research dam removal, fish

ladders and fish elevators, and come to a consensus about what they would do in one of the three dams.

Objectives: define fish ladder. Design a fish ladder. Calculate slope.

Materials: Maps with coordinates of dams, data specs for dams, photos, information about dam removal, fish ladders,

historical information about dams and specific dams.

Scenario:

It starts out as just another typical day in the Bronx. You wake up, get ready for school, hop on the train and away you

go. That afternoon you notice something very different as you walk back to the station. You can see the station, even

hear the train, but somebody has built a wall around the entrances and it is too big to jump. There is no other route

home except to get on that train. What will you do?

Allow students to brain storm methods of surpassing the barrier. At first any method is acceptable, then you impose

the following restrictions: You cannot use your hands, the wall has become a part of the train station so destroying it

will possible collapse the station, and/ or you have no tools. When the student feels the unfairness of the situation,

explain that this is just what has happened to anadromous fish like the alewife ever since the first dam was built. On the

Bronx River, the oldest recorded dam was built in the 1680s, in the same area where the dam is today north of 180th

street and is believed to have powered a grist mill and a sawmill. The stored water may also have been used to irrigate

the areas to the south which we know today as West Farms. (These areas may have had springs to irrigate, but I would

consider it likely that this water, held safely above the tides and the salt, would have been a great resource) The early

date of the first dams means that diadromous species would have been removed from the ecosystem, that is, if they

were ever there in the first place. The records prior to that time do not give such data for the Bronx River, however,

alewife do still run in undammed waterways in the area or had been noted as doing so in the past so it is quite most

likely true that they ran in the Bronx River as well.

Below are the three dams that exist in the Bronx at the present time.

Three Dams on the Lower Bronx River

Figure 1 Dam in River Park 13 feet in height

Figure 2Twin Dams at the Bronx Zoo 10 feet in height

Figure 3 The Snuff Mill Water fall at the NY Botanical Gardens 7 feet in height

Stairway to Spawning: An exercise in Fish Ladders

What does it mean to build a fish ladder? Next time you walk up a set of stairs, take a close look and ask yourself a few

questions: What height is each step? What is the depth of each step? Do the steps vary in height and depth? Why do

they have the measures that they have? Once you have thought about these questions, you will understand that the

design of most stairs follow certain rules that allow the majority of people to safely ascend and descend. The same is

true for fish ladders or perhaps we should call them fish steps as they do in Australia.

The velocity of falling water can be overcome in short bursts by healthy fish. Dams add the problem of height which

cannot be overcome. Alewife are capable of swimming against strong flowing water without much stress, but they are

not considered jumpers the way salmon are often photographed. If they do jump, it is very stressful on their physiology

and there chance of survival is lessened. The job of the fish ladder, as it is with stairs, is to reduce the need for sustained

effort and break the height up into workable, flowing units with space for rests in between.

Compare the steps below (2-4). They all reach the same goal of providing support for climbing the wall, but which one

would make the climb, both up and down, easier? The 8 ft high wall at the right is very steep, making ascension quite a

struggle. As you move farther away from the wall as you would with the bottom of a ladder, the slope of your climb

decreases making the climb require less effort. An actual ladder for a human to climb would be fine at about four feet

away from the wall, but it would not be safe for actual stairs. As you continue to move away from the wall, the slope

becomes more and more gradual until a person could actually walk up a gentle incline. In the example shown, the stairs

have been sketched in at eight feet. At eight feet away from the wall, with every one foot forward (run) you will go up

(rise) one foot. To calculate the slope, it is written in the form of a fraction with the rise over the run, here 8/8. Then to

find the percent of slope you divide the numerator by the denominator and multiply your answer by 100. For a rise of

eight feet and a run of eight feet the slope is 1 or 100%. In an actual staircase, you will find that the percent of slope is

usually between 77 and 80%.

Figure 4 Calculating slope for an 8 ft dam

In order for an Alewife to successfully navigate upstream the flow velocity must be less than ½ foot per second. This

can be broken down to a slope of between 10% and `15% grade. That means extending the bottom of the ladder

farther and farther away from the bottom until you reach the desired 15%. For an eight foot dam that would mean

about 53 feet of run from the foot of the dam to the bottom of the fish ladder. Using the Pythagorean Theorem you

can then calculate the actual length of the ladder. For this ladder it would be about 53.6 feet.

But what if you don’t have that much space to use? Just like in an apartment building, the structure can turn to one

side or the other, turn back upon itself or even spiral.

Figure 5 Spiral staircase Figure 3 Stairs with turn

For the present activity you will need to design a fish passage for at least one of the three dams still in place today along

the Bronx River in the Bronx. Using the data for the dams given above, calculate the run necessary to give the slope

that is needed for the fish ladder. Information should be presented in a visual form and include a drawing or a physical

model of the fish ladder that you design. Below are resources that you may find useful in your design quest.

River Damming and Fish Passage Resources

1) River and Dam information

http://www.internationalrivers.org/en/node/477

2) Historic fish passage discussion from Mass.

Historic Records Related to the Anadromous Fisheries of the Presumpscot River and Sebago Lake, Maine.

http://www.friendsofsebago.org/historicreport.html 3) Smithsonian Lessons Industrialization and the Environment

http://invention.smithsonian.org/centerpieces/whole_cloth/u2ei/

Sargent's Trench Trial is a good connection for damming issues

4) Photos of Fish Ladders

http://www.photolib.noaa.gov/habrest/crp_acc.html

5) Fish Ladder Design

http://www.environnet.in.th/eng/evdb/science/technology/01.html

http://www.michigan.gov/dnr/0,1607,7-153-10364_52259_19092-46291--,00.html

6) Alewife migration and fish passages

http://www.unity.edu/facultypages/aphillippi/alwife.htm

7) U.S Fish and Wildlife Dam Removal and Fish Passage Information

http://fpdss.fws.gov/

8) Anadromous fish and Habitat Restoration Methods

http://www.edc.uri.edu/restoration/html/tech_sci/restana.htm

9) Fish Ladder Article and Video

http://www.wral.com/news/local/story/3172802/

10) A Fish Elevator! Hadley Falls Fish Lift

http://www.hged.com/html/hadley_falls_fish_lift.html