Question 104·Medium·Command of Evidence
Because dams can block access to upstream spawning habitat, removing dams is sometimes used as a way to help migratory fish populations recover.
In a report on river restoration, a student claims that the removal of two small dams on the Clearfork River was followed by a sustained increase in the number of returning Chinook salmon.
Which choice best describes data in the graph that support the student’s claim?
For “Which choice best describes data” questions, translate the claim into a specific pattern you must see (here, higher values after the dam removal that stay high). Then scan the graph for the key comparison (before vs. after the marker) and pick the choice that states that comparison accurately without adding extra claims (like a perfectly steady increase) that the graph does not show.
Hints
Focus on “sustained increase”
Look for evidence that the numbers are higher for several years in a row, not just for one year.
Compare the two time periods
Use the dam-removal marker to split the graph into “before” and “after,” and summarize the typical range in each period.
Eliminate choices that describe the wrong trend
Check whether any option wrongly claims the graph rises smoothly every year or suggests there was no jump right after removal.
Step-by-step Explanation
Identify what the claim requires
The claim says dam removal was followed by a sustained increase, so the best evidence should compare salmon returns before and after the removal and show the higher numbers continue for multiple years.
Read the values before dam removal
From 2010 to 2015 (before the vertical marker for dam removal), the plotted points are all close to about 20–24 thousand returning salmon.
Read the values after dam removal
From 2016 to 2022 (after the marker), each year’s point is much higher—roughly 34 thousand up to the mid-40-thousands.
Choose the option that matches the before/after pattern
The option describing low 20-thousands before 2016 and about 34 thousand or higher in every year from 2016–2022 best supports the claim.
Correct answer: The number of returning salmon stayed in the low 20-thousands from 2010 through 2015 but was about 34 thousand or higher in every year from 2016 through 2022.