Something like a Scandinavian Lake District, Shumarinai-ko (朱鞠内湖) was created in 1943 by the building of Uryu Dai-ichi Dam (雨竜第一ダム) when Japan had entered into a wartime, along with many sacrifices.
This picturesque artificial lake was designed as a water-based prefectural natural park in 1974, consists a lot of bays, islands and peninsulas. The best way to experience the lake is to pack camping gear, rent a boat, fishing and spend a silent night.
Fishing is available from 1 May to 10 December (trout season: dawn-dusk), and from 1 January to 10 April (ice fishing season: 6am-4pm). You can borrow fishing rods and purchase a fishing ticket (¥1100) at the fisheries cooperative.
Note that the road to the another artificial lake Utsunai-ko (宇津内湖) is currently closed.
Points of interests around Shumarinai-ko
Shumarinai (朱鞠内) is served by infrequent JR buses from JR Fukagawa Station (7 daily; 73min) required a change at Horokanai (幌加内), as well as JR Nayoro Station (4 daily; 2hrs). Japan or Hokkaido rail passes are available.
From June to August, two buses between Nayoro and Horokanai continuing to Kohan (湖畔) stop on the shore of Shumarinai-ko.
If you travel to Shumarinai-ko by bus between October and May, get off the bus at Mitsumata (三股: three-forked road) stop, from where it is about a 1.5km walk to the shore.
More info www.jrhokkaidobus.com
Shumarinai Sightseeing Cruise
(朱鞠内観光汽船) From June to September, you can hop aboard 35-minute scenic cruise on the lake. There are also motorboats (¥4000 up to four people) and lowing boats (¥600 per 30min, ¥1000 per hour: up to three people).
Lake Shumarinai World Center
(シュマリナイ湖ワールドセンター) This NPO offers private fishing tours on the lake by boat (from ¥35,000 up to two people) from July to September, as well as irregular package tours (ice fishing, snowmobiling and lodging) in winter. They are the sole agent for the accommodation Lake House Shumarinai (レークハウスしゅまりない).
More info en.shumarinai.jp
(朱鞠内湖畔キャンプ場) On the peaceful shore of Shumarinai-ko, there are three seasonal campsites and year-round cabins, with showers and washing machines. Tent (from ¥1500 per night), sleeping bag and blanket rentals are available.