The cruised Mekong is not a single national river. Voyages run between the Vietnamese delta and Cambodia, so the trip includes a river border crossing with immigration formalities carried out on the water — an ordinary part of the itinerary here and not an incident.
The delta end is a maze rather than a channel. The river splits into many branches, and much of what there is to see — floating markets, brick kilns, workshops on stilts — lies down waterways too narrow for the vessel itself. Going ashore is therefore often done by small local boat rather than by walking off a quay, which makes the excursion pattern quite different from a European river.
Upstream the river is strongly seasonal. Flow rises hugely in the wet season and falls in the dry one, and the Tonlé Sap tributary reverses its direction of flow as the main river rises. How far a vessel can travel, and whether the lake stretch towards Siem Reap is reachable by water at all, depends on where the season sits.