Reference water levels
The reference water levels are used on inland waterways to define a range of water levels allowing the full use of the waterway for navigation. Ship passage can be limited by the water levels that are too low, when the fairway might become too shallow for large ships, or too high, when it might become impossible for the target ships to pass under the bridges. The goal of establishing the reference water levels is to balance the safety of navigation and economic value of the waterway. Reference levels are set up based on statistics obtained from the multi-decadal observations.
Free-flowing rivers
Free-flowing rivers use the low navigable water level as a low reference water level. When the river is at or above LNWL, the ships of the target draft can use its fairway. LNWL is usually defined as the water level that the river surface stays mostly above at the times when the river is navigable, so that the river level on average stays below the LNWL only for a fixed small portion of a year. For example, in Europe the limit of time below LNWL is 20 ice-free days per year or, alternatively, corresponds to 5–6% of the ice-free period on European rivers.The highest navigable water level is defined as a water level that is exceeded for only a few days a year.
Regulated rivers, canals
The low reference level is usually not defined for regulated rivers and canals, as in these cases the depth of the navigation channel is guaranteed by design.The high reference water level is sometimes set to accommodate the short-term variations and the effect of water level changes due to the operation of locks and weirs.