This is the 9153rd most frequent Russian word.
селенгинской
"Селенгинской" is the adjectival form of "Селенга," referring to something related to the Selenga River or the Selenga region.
Here, 'селенгинской' serves as an adjective modifying 'долины' to specify a location or region.
Мы посетили выставку, посвященную культуре селенгинской долины.
We visited an exhibition dedicated to the culture of the Selenga Valley.
In this sentence, 'селенгинской' is an adjective describing 'реке', relating it to the Selenga River.
Селенгинской реке приписывают множество легенд и преданий.
Many legends and tales are attributed to the Selenga River.
Here, 'селенгинской' functions as an adjective modifying 'области', referring to a geographical area.