Here’s the sentence structure:

• Over the next few years, (Prepositional phrase modifying the clause)
• increasing demands (Main Subject – “demands”)
  • on the Chattahoochee River, (prepositional phrase modifying “demands”)
    • which flows into the Apalachicola River, (relative clause modifying “River”)
• could alter the saline content of Apalachicola Bay, (Main verb – “could alter”)
  • which would rob the oysters there of their flavor, (relative clause. Logically needs to modify the preceding clause)
  • and to make them (logically, needs to be parallel to “rob”)
    • decrease in size,
    • less distinctive,
    • and less in demand.

The sentence says that:

Over the next few years, increasing demands on the C River could alter the saline content of the A Bay. This altering of the saline content would robe the oysters in the bay of their flavor and make the oysters smaller, less distinctive and less in demand.

The sentence has the following errors:

1. From the meaning, it is clear that “which would rob…” needs to modify the preceding clause. However, grammatically (on GMAT), ‘which’ cannot refer to the preceding clause.
2. “rob” and “to make” are not parallel.
3. “make them decrease in size” is a clumsy and non-standard way. The same idea can be communicated in better ways such as “decrease their size” or “make them smaller”. However, since this part is followed by adjectives for ‘oysters’ — less distinctive and less in demand, the correct form will be “make them smaller”, and not “decrease their size”.

Please observe that in the non-underlined part, “could” has been used to talk about a future possibility. When we want to talk about a hypothetical future, we can use conditional words such as could or would.