、、。
So to start: my main area of activity is the lower reaches of the Yoro River.
Literal
First, I [gen-の] activity-range [topic-は], mainly Yoro-river lower-reach-area [copula-です].
A textbook X は Y です self-introduction pattern, with は marking the topic and です closing with the polite copula. おもに 'mainly' is an adverb that scopes narrowly to the following noun phrase — 'mainly the lower reaches' (not 'mainly is the area'). 下流域 is a compact sinitic compound: 下流 'downstream' + 域 'zone/area,' the productive pattern behind much Japanese technical and geographic vocabulary. The Yoro River is in Chiba Prefecture.