Nepal, east lowlands - huge dry river beds ("washes") coming down from the Himalayas. Have they always been dry? Are t

Nepal, east lowlands - huge dry river beds ("washes") coming down from the Himalayas. Have they always been dry? Are they just dry in the dry season before the monsoon? No water in many of these river beds even makes it to northern India, surprising me. But it could always have been like this.