sorry, just noticed that you asked for a day trip.
the only day trip in the area is in colorado canyon, upstream from lajitas. I think its about 9 miles. if it were me, and i have a few days to kill, I would hit one of the loop hikes, or hit several of the day trip hikes, and forget the boating. ( forgive me) but one day just isn't enough time on the river, and IMO, you could see some wicked cool areas on foot.
One option, that would be WELL worth the time, and money, would be to rent a duckie from desertsports, take it to the terlingua creek observation area, and paddle upstream to fern canyon. the water is usually low enough that this is done more than you would think. you could easiliy spend a day in fern canyon., and do some other hikes in the area. make sure your tank is full, all the time.
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