Tenryugawa river originates from Lake Suwa in Nagano prefecture and runs to the south roughly along the boundary between Aichi and Shizuoka prefectures with merging many tributaries from Kiso and Akaishi mountain ranges. The total length is 213 km and the coverage is 5,090 km².
Tenryugawa river runs along the Median Tectonic Line (MTL) which changes its strike from E-W to N-S in this area. To the west of MTL, granitoids and metamorphic rocks of the Ryoke belt are widely distributed. On the other hand, high-pressure type metamorphic rocks of the Sanbagawa belt (including Mikabu Greenstones) and acceretionary complex of the Jurassic Chichibu belt and the Cretaceous to Tertiary Shimanto belt are distributed to the east of the MTL.
Tenryugawa has repeatedly caused a flood to its valley and has been called "Wild Tenryu". Steep topography and fragile geology make abundant sediment supply. Since the Meiji era (late 19th century), many dams have been built for flood control. However, reduction of sediment supply recently causes beach erosion (retrieve of a coastal line) at the mouth of the Tenryugawa river.

Tenryu River rocks

Shimanto accretionary belt rocks
Rocks of the Cretaceous to Tertiary Shimanto accretionary complex, composed mainly of sandstone and mudstone associated with blocks or lenses of chert, greenstone and limestone. Acceretionary complex is formed by accretion of the off-scraped materials of the subducting oceanic plate to the bottom of the continental plate. The majority of accreted materials is terrigenous clastic such as sand and mud however, a small amount of oceanic crust materials (chert, greenstone and limestone) are also off-scraped and accreted. Therefore, a small amount of these oceanic origin rocks are also found in accretionary complex incorporated in sandstone and mudstone.

Sanbagawa metamorphic rocks
Rocks of regional metamorphism origin. Regional metamorphism took place at the deeper part of the subduction zone than that of accretion, hence mineral assemblage of these rocks are changed to those stable under high-temperature and high-pressure conditions. Since regional metamorphic rocks were formed under strong shear stress at the subduction zone, they have strong planer (and/or linear) fabrics. These three rocks have undergone the same formation processes, however difference of protolith produced three different types of metamorphic rocks (psammitic schist: sandstone origin; siliceous schist: chert origin; greenschist: basalt origin).

Plutonic rocks
Igneous rocks with slow cooling rate of magma at the deep crust. These rocks are derived from the Ryoke belt widely distributed in the upstream of the Tenryugawa river.

Other rocks
Mylonite
A sort of fault rocks formed by shear deformation of a fault. Faulting under low-temperature (< 300 °C) condition causes brittle deformation to form cataclasite whereas faulting under higher-temperature condition causes ductile deformation to form mylonite. This mylonite is derived from the Median Tectonic Line that marks the boundary between the Sanbagawa and the Ryoke belts.

Quartz vein
A flush of high-temperature fluid in a host rock often causes crystallisation of elements dissolved in the fluid to form mineral veins. This quartz cobble was a part of vein formed by hydrothermal activity.

Greenschist
Greenschist was originally a basalt, the main constituent of the oceanic crust, which subducted and metamorphosed under high-temperature and high-pressure conditions. It has strong planer and linear fabrics by alignment of platy minerals such as chlorite and stick-shaped minerals such as actinolite and epidote under strong shear deformation during subduction.
(top: plane-polarised light, bottom: crossed polars; Act: actinolite, Chl: chlorite, Ep: epidote, Pl: plagioclase).

Mylonite
Mylonite is a sort of fault rocks formed by ductile shear deformation under high-temperature condition. Quartz grains are sheared and recrystallised as fine-grained aggregates whereas more rigid plagioclase remains as a large grain (called as a porphyroclast) that shows anti-clockwise rotation by sinistral sense of shear.
(top: plane-polarised light, bottom: crossed polars; Pl: plagioclase, Qtz: quartz).

Bulk sand

Similar to the Fujigawa river, quartz and feldspars are dominant. Quartz is derived mainly from sandstones and granitoids, albite (Ca-free plagioclase) is from low-grade metamorphic rocks of the Sanbagawa belt, and other (Ca-bearing) plagioclase, K-feldspar and micas are considered to be derived from Ryoke granitoids.

Heavy Minerals

Heavy minerals in the Tenryugawa river is quite varied. The most dominant mineral is ilmenite, which is followed by roughly equal amounts of hornblende and epidote. Ilmenite, hornblende, tourmaline, zircon and monazite are from Ryoke granitoids, epidote actinolite and chlorite are from Sanbagawa metamorphic rocks, respectively. There are some minerals characteristic to peridotite such as chromite and olivine, but peridotite (serpentinite) is not widely distributed in the valley of the Tenryugawa river. A small bodies of peridotite are found in the Mikabu belt around Hyogoe-pass, which is a possible source of these minerals. A few grains of cummingtonite were also found. The Ikuta granitic body (located in the northeast of Iida, Nagano) is known to contain cummingtonite, that is the most probable source of cummingtonite.

Zircon U-Pb ages
Most zircons have an age between 50 to 100 Ma, suggesting Ryoke granitic rocks origin. There are also a few grains of older ages, which are considered to be detrital zircons derived from sandstones of Chichibu and Shimanto Groups.