Why water in sea and river evaporates when it not reaches boiling temperature?
You are confused between Evaporation and Boiling. Evaporation is one of the two forms of vaporization. It is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimates) gain sufficient energy to enter the gaseous state. It is the opposite process of condensation. The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore, proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. higher vapor pressure). EVAPORATION OCCURS AT ALL Temperatures. This must answer your question.
