Geography – The southern end of the Sanriku Ria Coast

This zone’s coastline is also characterized by deep inlets forming the Sanriku Ria Coast. Beginning around Miyako and extending all the way to the Oshika Peninsula, the Sanriku Coast transitions from coastal terraces in northern Iwate to a ria coastline in the south. What’s the difference? The fault lines in southern Iwate, where the Sanriku Ria Coast is observed, run from north-northwest to south-southeast, whereas the coastline curves southwest at a steep angle. The fault lines have eroded, leading to the development of valleys that are relatively perpendicular to the sea. When sea levels rose after the end of the Ice Age, seawater entered these valleys, forming the inlets. In the northern parts, fewer valleys lead to the sea, and the rate of uplift of the land was faster.

The southern part of Iwate, roughly south of a line connecting Kamaishi to Morioka, has Nambu Kitakami Belt, consisting of geological strata from approximately 500 to 200 million years ago. Around 400 million years ago, parts of the vast Gondwana supercontinent, which was located in the southern hemisphere at the time, including accretionary bodies, shifted to their present positions. This area is a relic of ancient times, where fossils of marine and coastal flora and fauna are also found.

Oshika Peninsula and Oshika Islands

The Oshika Peninsula is located at the southern end of the extension of the Kitakami Mountains. The surrounding area is one of the regions with the highest number of islands among the ria coasts. Among them, Kinkasan stands out as the largest, and it is revered as a sacred site along with Osorezan and the Three Mountains of Dewa, collectively known as the Three Spiritual Places of Oshu (Tohoku region). Other than priests, there are no residents there. The second largest island is Ajishima Island, which is known for its long history as seen in Jomon pottery found there. Similar to the Oshika Peninsula, Matsushima Bay was formed by the rise in sea levels during the most recent interglacial period. At the entrance of the bay, there are numerous small islands such as Miyato Island, Kanpuzawa Island, Nonoshima Island, Katsurajima Island, among others, creating a diverse archipelago. During the last glacial period approximately 10,000 years ago, these islands were part of the mainland beyond the current coastline.

Kitakami River, the largest river in Tohoku

The area in between the Oshika Peninsula and Matsushima is Ishinomaki. It is located on a delta developed at the mouth of the largest river in Tohoku, the Kitakami River. The Kitakami River flows straight south through the Kitakami Basin, situated between the Kitakami Mountains and the Ou Mountain Range. Its source is around halfway between Morioka and Ichinohe, near the northern part of Iwate Town, and its total length from the source to the mouth is 249 km. Almost all major cities in the inland areas of the prefecture developed along this river. Various products from within the prefecture, including rice, were transported to Ishinomaki via the Kitakami River’s water transport and then farther to Sendai and Edo (Tokyo today). The historical Oshu Kaido route extends along the Kitakami River from Ichinoseki to the north, forming a major artery of transportation in Iwate, combining both water and land routes.

While the Kitakami Mountains are composed of hard geological formations originating from ancient continents, the Ou Mountain Range consists of softer geological formations created by later uplifts and volcanic eruptions. Therefore, sediment tends to accumulate on the right bank of the Kitakami River, on the side of the Ou Mountain Range, causing the river to develop as if being pushed by the Kitakami Mountains side, and there are many major tributaries on the Ou Mountain Range side.

The mouth of the Kitakami River was originally a floodplain with an undefined course; therefore, flood control and channel relocation have been carried out since the time of Date Masamune. The river course, originally merging into the Hazama River in the northern part of Tome, was dammed up to prevent flooding in Tome by diverting it to the Futamata River. Later, to enhance the water transport of the Kitakami River, the Kitakami River, Hazama River, and Eai River were merged to secure a route to Ishinomaki. However, flooding occurred upstream from the confluence point. Since the Meiji era, most of the flow has been diverted to the Oshika River (formerly known as Oppa River), and that part has been renamed as the Kitakami River. The former Kitakami River passes through the city of Ishinomaki. As a result, a rational flood control system was completed, diverting the mainstream away from Ishinomaki toward the other side of the Oshika Peninsula while ensuring a shipping route from the upstream to Ishinomaki.

A Bay with a Rich Nature

Shizugawa Bay, facing Minamisanriku Town, was registered as a Ramsar Convention wetland in 2018. The coastal area of Minamisanriku is characterized by the unique convergence of warm currents (Kuroshio) and cold currents (Oyashio). As a result, Shizugawa Bay harbors a rare seaweed bed where wakame, which grows in warm waters, and ma-konbu, which grows in cold waters, coexist, creating a globally unique, rich ecosystem. This environment supports a diverse range of species, including endangered species such as the black-faced spoonbill, white-tailed eagle, and Steller’s sea eagle, which gather here for wintering.

Adjacent to Shizugawa Bay to the south is the Oppa Bay. Here, abundant plankton carried by the Kitakami River attract fish and shellfish, which in turn attract birds, creating another rich natural ecosystem. Nagatsura Bay is a tidal flat lake where seaweed and oysters are cultivated extensively. The warm-temperate plant community on Hakkeijima Island at the mouth of the bay, including camellias and Japanese blue oaks, is designated as a natural monument. The area is also famous for its fishing grounds.

Town

Fishing towns along the southern Sanriku Ria Coast

The Sanriku Ria Coast is dotted with towns developed from fishing villages nestled in inlets and bays. Along the southern part of the Sanriku Ria Coast, the towns are Kesennuma facing Kesennuma Bay, Minamisanriku, which faces Shizugawa Bay (a town that came into being by merger of Shizugawa, Utatsu, and Togura), Kitakami Town Jusanhama facing Oppa Bay, Ogatsu with the Ogatsu Bay; and Onagawa with the Onagawa Bay. Onagawa is located right at the base of the Oshika Peninsula. Beyond the coast are towns along the Oshika Peninsula, where fishing villages have also emerged in the inlets and bays. The eastern side of the peninsula faces the Pacific Ocean, while the western side faces Sendai Bay. Due to the presence of large ports such as Ishinomaki and Shiogama on the western side, which connect to Sendai and Edo (Tokyo), the towns on the western side of the Oshika Peninsula have prospered. From the tip of the peninsula, including Ayukawa, Kobuchihama, and Hagihama, fishing villages line the inlets, which have been active fishing grounds for a long time. Ayukawa, in particular, has a history of active whaling since ancient times. This area has the highest concentration of inlets and islands along the Sanriku Ria Coast, with many rocky reefs, making it an especially rich fishing ground along the coast.

Ishinomaki, the largest fishing port in the Tohoku region

The second most populous city in Miyagi Prefecture, Ishinomaki is located just west of the scattered settlements on the Oshika Peninsula. In fact, due to recent mergers, Ishinomaki City now encompasses Jusanhama, Jusanhama, Ogatsu, and most of Oshika Peninsula. Positioned at the mouth of the Kitakami River, the largest river in Tohoku, Ishinomaki historically thrived as a hub for shipping boats from major towns along the Kitakami River. During the Edo period, rice from Hachinohe Domain, Morioka Domain, and Ichinoseki Domain was consolidated here before being transported to Sendai and Edo. Known historically as Oshikano Minato, it was also referred to as Ijino Minato or Ishino Minato, which is said to be the origin of the name Ishinomaki. During the Edo period, the East Sea Route was developed, connecting Sakata on the Sea of Japan side to Choshi and Edo via the Tsugaru Strait, with large ships stopping at Ishinomaki and Arahama (at the mouth of the Abukuma River) on their way south. Ishinomaki is a city of maritime trade and fishing. It is designated as a Specific Third-Class Fishing Port, one of thirteen such ports nationwide, capable of accommodating nationwide use. Among these ports, four are located in the Sanriku region, including Kesennuma, Hachinohe, Shiogama, and Ishinomaki. Ishinomaki boasts the highest volume of fish landings among the four.