Fish Ladders Background Information The history …

1 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.

2 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.

3 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 ( ) and arguments against removal ( ) 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.

4 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.

5 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.

6 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.

7 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 2 Twin 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.

8 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.

9 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.

10 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 foot per second.