A low water crossing (also known as an Irish bridge, causeway in Australia, low level crossing or low water bridge) provides a bridge when water flow is low. Under high flow conditions, water runs over the roadway and precludes vehicular traffic. This approach is cheaper than building a bridge to raise the level of the road above the highest flood stage of a river, particularly in developing countries or in semi-arid areas with rare high-volume rain.
A simple low-water crossing can be constructed with culverts. Culverts (often concrete pipes) are used to carry the water in a stream keeping the crossing surface dry for most of the year. High flows, i.e. spring runoff or flash floods, flow over the top of the crossing, as the culverts are not large enough to carry these flood-type runoff events.
A more elaborate low-water bridge will usually be an engineered concrete structure. There are thousands of such structures in the western United States; some of them accommodate four-lane city streets or highways. Typically, a low-water bridge that accommodates a high daily volume of vehicular traffic will be underwater only a few days per decade.
A low-water bridge is sometimes called a submersible bridge, but this is a misnomer. A true submersible bridge is used on navigable waterways and is actively lowered into the water.
Despite the obvious dangers and warnings given there are still a significant number of emergencies and even deaths due to unwary use of low-water bridges during flood conditions.