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What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}

Here is a real image of the phenomenon. _{(image from Spectacular tidal bore surges up China river)}

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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.

_{(image from Fisheries and Oceans Canada - Tidal phenomena - Shallow water effects)}

Here is a real image of the phenomenon. _{(image from Spectacular tidal bore surges up China river)}

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}

Here is a real image of the phenomenon. _{(image from Spectacular tidal bore surges up China river)}

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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 - due to the small slope and narrow width - in a sudden wave-like rise of the water-level.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}

What you are describing is called a tidal bore.

Quoted from Wikipedia - Tidal bore - Description:

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.

_{(image from Fisheries and Oceans Canada - Phenomena - Tidal bores)}