Iwase River

...The river originates in the mountains at an altitude of about 500m in the southern part of the Miyakonojo Basin, and flows north through the basin, joining the Hagiwara River, Okimizu River from the east, and the Yokoichi River and Shonai River from the west. It erodes the mountains at an altitude of about 350m in the northern part of the basin for about 10km, drains the Kobayashi Basin to the west, joins the Iwase River flowing east, turns east, passes through Takaoka Town, forms the wide alluvial plain of the Miyazaki Plain, and flows into the Hyuga Nada Sea after joining the Honjo River flowing from the northwest in the west of Miyazaki City. Miyakonojo City and Kobayashi City are located upstream, and Miyazaki City is located downstream. The total population of the cities and towns in the basin accounts for about half of the total population of the prefecture, making it the central river of the prefecture.

*Some of the terminology that refers to "Iwase River" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…上流は都城（みやこのじよう）盆地南部の標高500m内外の山地に源を発して，東側から萩原川，沖水川，西側から横市川，庄内川などを合して盆地内を北流する。盆地北部の標高350m内外の山地を約10kmにわたって浸食し，西方の小林盆地を排水して東流する岩瀬川を合わせて東に曲がり，高岡町を経て宮崎平野の広い沖積地を形成し，宮崎市西部で北西から流れる本庄川を合流して日向灘に注ぐ。この川の上流には都城市，小林市，下流には宮崎市があって，流域の市町村人口の合計は県総人口の約半分を占めており，県の中心的河川である。…

※「岩瀬川」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報