English: Description at USGS website: "
Undular bore and
whelps near the mouth of
Araguari River, Brazil. View is oblique toward mouth from airplane at approximately 100 ft. altitude. Portion published as Figure 5 in
U. S. Geological Survey, Circular 1022, 1988." Note: This undular bore (locally known as
pororoca) is on the Araguari River in the
Amapá state in north-eastern Brazil, see
w:Tidal bore#South America. The bore propagates from left to right, against the stream of the river current.