Earth and Space Sciences Regents, August 2025, question 39
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What the figure shows
The model River Ecosystem with a Dam shows a reservoir behind a dam, with an inset labelled Barriers to migration and Loss of sediment transport, and a label Creates algal bloom on the reduced flow below the dam. Two diagrams show a river before and after it was dammed, with arrows showing flow from upstream to downstream and grey circles marking fish spawning and rearing habitat. Free-Flowing River: a winding river with several branches, with many circles along the main channel and its branches. River with Dam and Reservoir: a dam at the downstream end and a wide reservoir that covers the old river channel, with three circles, each on a small stream where it enters the reservoir.
How to think about it
The question asks about the area above the dam, so look at what happened upstream, behind the dam. Compare the number of spawning circles in the two diagrams and notice what now covers the old river channel. Then follow the chain: fewer or more places for fish to spawn leads to fewer or more fish to catch.
Based on the River Ecosystem with a Dam model and the diagrams, which evidence-based statement correctly identifies how the availability of fish-spawning habitats has influenced commercial fishing above the dam?
Choice 1 is right. A dam holds water back, so the water level upstream rises and a reservoir floods the old river channel. In the diagrams, the many spawning sites along the free-flowing river shrink to three sites at the edge of the reservoir. Fewer places to spawn means fewer young fish, so fewer fish are available for commercial fishing above the dam.
This choice is about the river below the dam, while the question asks about the area above it. The model also gives no evidence of more dissolved oxygen, and more oxygen would improve water quality, not lower it. This choice may come from linking the dam to a change in water quality below it without checking which side of the dam the question asks about.
A dam raises the water level upstream; it does not lower it. The diagrams also show far fewer spawning sites after the dam, so fish spawning decreased. This choice may come from picturing the reduced flow below the dam instead of the reservoir behind it.
This choice is about the river below the dam, while the question asks about the area above it. Less dissolved oxygen lowers water quality for fish, so this chain of effects does not hold. This choice may come from thinking of dissolved oxygen as a pollutant, so that less of it would make the water cleaner.
