A personal illustration how those hydronymic things work. I live in Saint Petersburg, a city founded in 1703 in the place of a Swedish fortress built among mostly Finnish villages. As of 2018, the name of the main river,
Нева/Nʲeva, is unchanged (it's etymology is unknown, it may be even Indo-European, with the meaning "new" — the Corded Ware culture was widespread not far from here:
Corded Ware culture - Wikipedia and the river itself emerged a few millennia ago when waters of the Ladoga lake found a new way to discharge into the gulf of Finland); only one noticeable river,
Охта/Oxta, still bears an unchanged Finnish name
Ohta, and its tributary
Оккервиль/Okkʲervilʲ may be named after a Swedish estate or may continue a very distorted Finnish word. The name of another noticeable river,
Карповка/Karpovka, looks Russian, but actually is a distorted Finnish
Korpijoki. Most other river names are new. The area not far from my house is called
Ульянка/Ulʲjanka, which looks derived from the Russian name
Ульяна/Ulʲjana (<
Juliana), but in reality is a Russified Finnish
Yljälä. The rivers in my district, from east to west, are
Красненькая/Krasnʲenʲkaja, Дачная/Dačnaja, Новая/Novaja, Дудергофка/Dudʲergofka, Ивановка/Ivanovka and
Сосновка/Sosnovka, of which
Dudʲergofka is named after a Swedish
Duderhof estate, others are Russian and new, but
Sosnovka in some Internet sources still has its aboriginal name
Mitkazi mentioned (apparently taken from one of non-Finnish Baltic-Finnic languages). Thus, for some 100 years, mostly in the course of the 18th century, with the influx of a new ethnic group, the old hydronyms first remained in use among the original population and then simply vanished.