ostium vs rift

ostium

• The mouth of a river. 

• A small opening or orifice, as in a body organ or passage. 

• Any of the small openings or pores in a sponge. 

rift

• A shallow place in a stream; a ford. 

• A break in the clouds, fog, mist etc., which allows light through. 

• A chasm or fissure. 

• To form a rift; to split open. 

• To belch. 

• To cleave; to rive; to split.