GMAT Sentence Correction (SC) | Around 1900, fishermen in the Chesapeake

Quick question:

In B, shouldn't "blocked" be in the past perfect? The block happened first, and then came the reduction?

Or is blocked OK because it's part of the noun modifying phrase that modifies milldams and culverts, and the only verb that connects to 1920 is reduced, requiring a past perfect?

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