What you are describing is called a tidal bore.

Quoted from [Wikipedia - Tidal bore - Description][1]:

> Bores occur in relatively few locations worldwide, usually in areas
 with a large tidal range (typically more than 6 meters (20 ft) between
 high and low tide) and where incoming tides are funneled into a shallow,
 narrowing river or lake via a broad bay. The funnel-like shape not only
 increases the tidal range, but it can also decrease the duration of the
 flood tide, down to a point where the flood appears as a sudden increase
 in the water level. A tidal bore takes place during the flood tide and
 never during the ebb tide.

In the ocean the sea level rises quite slowly (only a few feet per hour).
But in the shallow water of rivers (and also in funnel-like bays)
this will result in a sudden wave-like rise of the water-level.
Due to the small slope of the river the slow vertical rise (a few feet per hour)
is converted to high horizontal speed (several kilometers per hour) of the wave-front.
And due to the funnel-like shape (from a wide sea bay to a narrow river)
the height of the wave front piles up on the way.

[![tidal bore][2]][2]  
<sub>(image from [Fisheries and Oceans Canada - Phenomena - Tidal bores][3])</sub>
 
Here is a real image of the phenomenon.
[![enter image description here][4]][4]
<sub>(image from [Spectacular tidal bore surges up China river][5])</sub>

  [1]: https://en.wikipedia.org/wiki/Tidal_bore#Description
  [2]: https://i.sstatic.net/aXPYI.gif
  [3]: https://www.waterlevels.gc.ca/eng/info/phenomena#s22
  [4]: https://i.sstatic.net/06tXK.jpg
  [5]: https://www.telegraph.co.uk/news/worldnews/asia/china/11083187/Spectacular-tidal-bore-surges-up-China-river.html