Also known as a tidal bore but here in Alaska called "the bore tide," this is a large wave created by an incoming tide working against the outgoing flow in the Turnagain Arm, a fjord off of Cook Inlet south of Anchorage. Because the ford is so shallow and narrow, the wave can be several feet high and quite dramatic. Incredible to watch, as this video on YouTube suggests.
Thanks, skipvia! I was delighted to be able to see a bona fide tidal bore this past summer, in the Bay of Fundy. Absolutely amazing. Unfortunately, I don't have a video camera, but the series of stills I took help me remember it. :-)