GMAT Sentence Correction (SC) | As rainfall began to decrease in the Southwest about the

I do not see any problem with "having". A present participle modifier such as this one is alright and is used as an adjective (in this case for the noun "clan"). The problem with D and E is the awkward usage "water access". "Water access" may mean "access BY water" ( e.g. land access, water access etc.) and may not mean "access TO water".