2．The river flows calmly in my town because it's located downstream.
｢下流｣=｢Downstream｣(副詞、形容詞)
1．｢私の町は川の下流にあります｣=｢My town is downstream.｣
下流→川の下流と理解してもらえるので、ここでは｢The river｣を省略しています。
Downstreamを形容詞として使っています。
2．｢私の町は川の下流だから流れは穏やかだよ｣＝｢The river flows calmly in my town because it's located downstream.｣
｢川が流れる｣＝｢Flow｣
｢川の流れ｣=｢A flow of the river｣ Flowは動詞、名詞ともに使えます。
｢～が位置する」｢～のところにある」=｢～be located｣
ここでは、Downstreamを副詞として使っています。
例; My house is located near Tokyo Station.(私の家は東京駅のそばにあります)
my town is located on the lower reaches of the river
(my town is located on) the lower reaches of the river so the water is very calm (and slow-moving)
「私の町は川の下流に位置しています」＝ my town is located on the lower reaches of the river
「私の町」＝ my town
「川の下流」＝ the lower reaches of a/the river
「位置しています」＝ be located (in, on, at)
「川の下流だから流れは穏やかだよ」＝ (my town is located on) the lower reaches of the river so the water is very calm (and slow-moving)
「流れは穏やか」＝ the water is calm / the water is slow-moving
日本 
イギリス 
