Navigation season

In the water transport, the navigation season (or shipping season) is a period of the year during which a waterway is open for navigation. The opening and closing dates of the season are determined annually by the waterway administration to accommodate the weather, water flow and ice conditions as well as the transportation demands. The term is also frequently used to designate number of days per year with easy navigation.

Terminology
In a general sense, in different contexts the term has slightly different meanings, multiple expressions are used (for the Northern Hemisphere):
 * summer navigation season;
 * winter navigation season with some waterways closed;
 * ice-free navigations season / independent navigation period when the ships can get through without the assistance from an icebreaker.

A traditional navigation season naturally varies from оne year to another, a fixed navigation season alternative sets firm dates for opening and closing (that can be changed based on conditions, but only to shorten the period), an extended navigation season utilizes technical means to continue navigation under adverse conditions (e.g., ice).

A navigation window is created by a short controlled release of water from an upstream reservoir to temporarily create a higher river stage for the purpose of navigation.

Antiquity
In Ancient Greece, the navigation period on the Mediterranean Sea lasted while the Pleiades star cluster was visible (May to November). Some interpretations of the Story of Wenamun (Ancient Egypt, circa 1000 BC) and texts from Ugarit (2nd millennium BC) suggest the existence of navigation seasons even earlier.

Europe
Navigation periods in Europe have a long history. Maritime law of Hanseatic League already in the 13th century stated that the ships must remain in the winter ports after St. Martin's Day (November 11). By the 15th century more detailed laws allowed sailing on the Baltic Sea from Cathedra Petri (February 22) to St. Martin's Day. The season in the North Sea was even shorter, from one Feast of the Cross (Invention of the Cross on May 3) to another (Exaltation of the Cross on September 14).

USA
Multiple Rivers and Harbors Acts left the administration of navigation on the major US waterways, including the commencement and closing of the navigation season, to the U.S. Army Corps of Engineers (USACE).