Further correct solutions were also received from Al Zimmermann, William Webb, Douglas Vander Griend, Brian Laughlin, Ivan Lisac, Philippe Fondanaiche, Michael Moyer, Emily, Thomas Teo.  Three incorrect solutions were submitted.

Voilà! No equations.

When the boats meet the first time, the total distance they have travelled spans the river once.  When they meet for the second time, they have spanned the river together twice more.  As the boats are travelling at a constant speed, the last distance covered by each of them is twice the distance covered previously.  So one of the boats has travelled 700 + 2*700 = 2100 feet.  Since the boats are 400 feet from the bank, the river is 2100 - 400 = 1700 feet wide.

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