I have no idea what the commentaries say, but having grown up in a place (and now I live) where there were a lot of "trees planted by the rivers of water" I always took it to be a generic example. Any time you plant a tree near water, it flourishes, even in times of drought.

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J Baldwin
Keowee Presbyterian Church, PCA
Pickens, SC
“You shall love the Lord your God with all your heart and with all your soul and with all your strength and with all your mind, and your neighbor as yourself.” Luke 10:27

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