Seems to be pretty evenly split on whether the dash is needed, so I'd use your personal preference there (i.e., perhaps "low water channel").

Estiaje clearly refers to low water, and there is very strong concordance through Google, just considering the quality of the government and NGO websites which use this term.

I would use that generally high quality of similar uses, such as the websites of government agencies, NGOs and scientific literature, to determine that this is an acceptable term.