Gauge Level and Water Depth: Two Different Measurements

A gauge reading and a water depth do not describe the same thing. A gauge indicates the water level at a fixed measuring point relative to the gauge datum. Water depth, by contrast, is the distance between the water surface and the riverbed at a particular location. Since the riverbed is not at the same elevation everywhere, a gauge reading alone cannot tell you the depth at every point along a stretch of river.

What a gauge reading tells you

A measurement applies to the gauge and time at which it was recorded. For navigation, gauge readings are also linked to specified reference water levels. This reference helps put stretches of river and published target depths into context, but it is not itself the water depth beneath a vessel.

One example is the German stretch of the Rhine: the Federal Waterways and Shipping Administration (WSV) assigns gauges to specific kilometre sections. Its guidance identifies the Kaub gauge for Rhine kilometres 540 to 566 and the Koblenz gauge for kilometres 566 to 601. Gauge coverage and the sections for which target depths are specified therefore do not always coincide.

Where the actual navigable-channel depth comes from

Depth depends on how far the water surface is above the riverbed. Shallow spots and deeper areas can vary within the same stretch. The WSV guidance lists values referenced to the Equivalent Water Level 2022 (GlW 2022) and describes the navigable-channel depth as a target depth maintained where possible and reasonable. It lists different values for different Rhine stretches, such as 1.90 metres between Budenheim/Niederwalluf and St. Goar, and 2.10 metres between St. Goar and Koblenz.

A target depth of this kind is therefore not a current measurement for every point in the navigable channel. The gauge describes the water level at its measuring point; the target depth applies to the specified stretch and reference water level. Local or temporary restrictions may differ from it.

How to interpret the figures correctly

To make a meaningful comparison, a gauge reading must be considered together with its measuring point and measurement time. For navigable-channel depth, the stretch, the reference water level used, and whether the figure is a target value or a current local report are essential. The information provider and the date of the information also help define what the figure means.

A single gauge reading proves neither the depth at an arbitrary location nor whether a particular vessel can navigate the route. That requires taking account of depth information for the relevant stretch and any operational restrictions. Distinguishing between gauge level and depth prevents a water-level figure from being mistakenly read as the clearance beneath a vessel or as a definitive statement about the entire route.