This is one of the two strategic gaps in the city wall of Ankh-Morpork, where the city and its encircling walls are bisected to allow for the passage of the River Ankh.
Situated at the hubwards end of the city, the Water Gate utilises a mole, or pier head, and a system of floating barricades which may in time of emergency be moved into place to completely block the river from bank to bank. Most of the time, though, the gap is left open to allow for the free movement of river traffic and commerce, such as the Zoon-crewed barges floating down from Zemphis, higher up the river.
It is also stated that much of the city's coal and iron ore imports from points further away, such as Lancre, also arrive by the river route.
In the opening paragraphs of The Truth, we learn that a Watch guard is always posted here by night to act as a not-very-efficient customs point to quiz vessels arriving during darkness as to their contents. As Mr Pin and Mr. Tulip were able to smuggle Charlie in this way, we know the Watch guard is not a particularly efficient one.
As with the River Gate which guards the far wider estuary end of the Ankh, the gate is only closed in time of great city emergency, such as a massive fire requiring the water level in the Ankh to rise so that any conflagration is eventually smothered and not so much extinguished as poisoned. The last time this was needed was quite a few decades before the consensus present, and is described in The Colour of Magic. For this to work, though, it would require the River Gate to be closed completely and pretty nearly watertight, whilst the Water Gate is used to regulate just the right amount of flow to enter the city so as to sicken the blaze. (In cities with less solid and polluted rivers, the word dampen might be validly employed).
At both Gates, there is a passenger ferry to allow direct access from one bank to the other (ie, across Endless Street at both points where it meets the River). Whether or not Ankh-Morpork has its song equivalent to Ferry 'Cross The Mersey (in response to a multiversal prompt) is not yet recorded.