[1] The goal of establishing the reference water levels is to balance the safety of navigation and economic value of the waterway[2] (for example, increase of the low level shortens the navigation season but allows the use of larger ships).
Reference levels are set up based on statistics obtained from the multi-decadal observations (typically 30 years).
For example, in Europe the limit of time below LNWL is 20 ice-free days per year (e. g., on Rhine) or, alternatively, corresponds to 5–6% of the ice-free period on European rivers (6% on Danube[3]).
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 (sufficient margins are covering the variations of the flow).
[1] In Germany, an equivalent water level (German: Gleichwertiger Wasserstand, GlW) is the value of a stream gauge that corresponds to the nominal fairway depth (ideal minimum channel depth, TuGlW).