the middle part of a river, stream, etc. 中流；河流中心 We anchored in midstream. 我们在河中心抛锚停泊。 The boat drifted out into midstream. 船漂到了中游。
Idioms
change horses in midstream
to change to a different or new activity while you are in the middle of something else; to change from supporting one person or thing to another 中流换马；中途变卦；转而支持另外的人（或事）
(in) midstream