Graphic organizer Grade 10 Engineering & Design
Burnthorpe Dam Cause-and-Effect Chain: Retrofit or Remove
A grade 10 cause-effect chain organizer tracing six consequences of a fictional aging river dam and pairing each one with the engineering response it calls for.
Objectives
- Students will trace a chain of consequences from a single dam to effects several steps downstream.
- Students will pair each effect in the chain with an engineering response and the evidence that would justify it.
- Students will state which response they would carry forward and what data would settle the choice.
Vocabulary
Key words: fish passage, sediment starvation, channel incision, multi-level intake, spillway capacity, staged removal
Teacher overview
This organizer is a linear cause-effect chain, read from left to right, with a decision box at the end. It follows six consequences of one fictional structure, Burnthorpe Dam on the fictional Glasswort River, from the physical change the dam makes to the river all the way to the choice a town has to make about it. The chain structure matters because dam effects are sequential rather than parallel, since trapped sediment causes downstream gravel starvation, which in turn causes channel incision, and a student who lists effects as an unordered set cannot see where an engineering response would break the chain. A frequent misconception is that a fish ladder addresses every problem an old dam creates, when a ladder does nothing about sediment, water temperature, or spillway capacity.
Organizer
Read the case notes, then complete every cell in the chain. The cause for each link is given in the second column. Links 2 and 3 connect, because the sediment trapped in link 2 leaves the river below sediment poor in link 3.
Case notes. Burnthorpe Dam is a fictional structure on the fictional Glasswort River. It was built in 1936 to power a mill that closed decades ago. The dam stands 42 feet high with no fish passage of any kind. The reservoir behind it has filled with sediment over 90 years. Water leaves through a low-level outlet near the bottom of the pool. The spillway was sized for a smaller design storm than the county now uses. The town owns the dam and must decide between a retrofit and a staged removal.
| Link | Cause at this link | Immediate effect | Effect that follows downstream | Engineering response to consider | Evidence that would justify the response |
|---|---|---|---|---|---|
| 1. Passage | The dam blocks the channel | ||||
| 2. Sediment | The reservoir slows the current | ||||
| 3. Channel | The river below runs sediment poor | ||||
| 4. Temperature | Water is drawn from the bottom of the pool | ||||
| 5. Flood risk | The spillway is undersized for the current design storm | ||||
| 6. Community | Either choice changes the river the town has known |
Decision box. Which response would you carry forward, and what single piece of evidence would settle the choice? Write two to three sentences.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
Case notes. Burnthorpe Dam is a fictional 42-foot structure on the fictional Glasswort River, with no fish passage, a sediment-filled pool, a low-level outlet, and an undersized spillway.
| Link | Cause at this link | Immediate effect | Effect that follows downstream | Engineering response to consider | Evidence that would justify the response |
|---|---|---|---|---|---|
| 1. Passage | The dam blocks the channel | Migrating fish cannot reach spawning gravel upstream of the dam | The upstream population declines and the reach above the dam loses the nutrients returning fish carry | Build a fish ladder or a nature-like bypass channel, or remove the dam | Counts of fish holding below the dam during the migration season, and a survey of usable spawning gravel upstream |
| 2. Sediment | The reservoir slows the current | Coarse sediment drops out and settles in the pool | The pool loses storage each year while the river below is starved of the gravel it used to carry | Sluice sediment during high flows, dredge the pool, or plan a staged removal with a sediment management plan | Bathymetric surveys of the pool over several years, and grain-size sampling of the trapped material |
| 3. Channel | The river below runs sediment poor | The bed coarsens as fine material washes away and the channel begins to cut down | Banks steepen and fail, and spawning gravel disappears from the reach below the dam | Add gravel to the channel below the dam, or restore sediment transport by removing the barrier | Repeat cross-section surveys showing bed elevation dropping, and photographs of exposed bank toes |
| 4. Temperature | Water is drawn from the bottom of the pool | Summer releases are colder and lower in dissolved oxygen than the natural river | Species adapted to the reach are stressed, and low oxygen limits what can live below the outlet | Install a multi-level intake to blend water from different depths, or aerate the outlet works | Paired temperature and dissolved oxygen readings above and below the dam through a summer, showing releases near 4.5 mg/L |
| 5. Flood risk | The spillway is undersized for the current design storm | Large storms would send water over the crest rather than through the spillway | Overtopping could breach the embankment and release the stored sediment all at once | Enlarge the spillway, add an auxiliary spillway, or remove the structure so the hazard ends | A hydrologic study comparing the spillway's capacity with the county's current design storm |
| 6. Community | Either choice changes the river the town has known | The reservoir either stays as flatwater or returns to a flowing channel | The town gains or loses reservoir recreation, a mill-era landmark, and a long-term maintenance obligation | Stage the removal with a sediment plan and an interpretive site, or fund the retrofit and its inspection schedule | A cost comparison of removal against 50 years of inspection, repair, and liability for the retrofit |
Decision box. Staged removal with a sediment management plan addresses links 1 through 5 at once, while a fish ladder addresses only link 1 and leaves the town holding the spillway and inspection obligation. The single piece of evidence that would settle it is the grain-size and chemistry sampling of the trapped sediment, because clean coarse sediment can be released in stages while contaminated fine sediment would have to be removed and disposed of, which can make removal the more expensive path.
Differentiation
Support
Give these students the chain with the Immediate effect column already filled in, so their work begins at the downstream column. Provide a word bank for the response column that lists fish ladder, bypass channel, sluicing, dredging, gravel augmentation, multi-level intake, spillway enlargement, and staged removal, and let them match rather than generate.
Extension
Ask these students to add a seventh link for what happens to the reach above the dam after removal, as the former pool drains and the old channel re-forms, and to name the response that protects water quality during that transition. Have them argue which links a fish ladder would leave completely unaddressed.
ELL supports
Provide a labeled side-view sketch of the dam showing the crest, spillway, low-level outlet, sediment wedge, and downstream channel, and keep it beside the organizer. Post the terms cause, effect, downstream, sediment, and passage with short definitions, and allow the decision box to be completed with a sentence frame beginning "I would choose ... because links ... and ... show ...".
Extensions
Have students find a dam in their own state that has been studied for removal or fish passage and map the agency documents onto the same six links, noting which links the real study emphasized and which it barely mentioned. Students can also interview a parks or public works staff member about a local impoundment and add a seventh column recording who would pay for each response.