The length of the Moika River is 4.67 km, the width up to 40 m, and the maximum depth up to 3.2 m. As can be seen on the terrain plan taken from an old Swedish map of the surroundings of Nienstadt/Nyenschantz (presumably from 1698), the river was an independent branch of the current Fontanka even before the founding of St. Petersburg.

On these maps, the left tributary of the Neva, now called the Moika River, is marked as pieni muya yoki, which means "little dirty river." In Russian land survey books, the original Finno-Ugric Muya ("dirty") was recorded as Mya, which later transformed into Moika. In Russia, until 1797, the river was officially called Mya, and the first mentions of Moika were recorded from 1726.

Joining the Fontanka at an angle close to a right angle, due to hydrodynamic laws, the source of this branch and its upper reaches gradually silted up, and due to the low natural slope, the entire surrounding area became swampy. A similar fate possibly befell a nearby channel, which was independent in the early stages of the Neva delta formation but later turned into the swampy Krivusha River. The process of swamp formation at the sources of such branches intensified with each flood in St. Petersburg, two of which occurred within five years after the city's founding (1703 and 1708). As a result, at the beginning of the 18th century, the Moika River was a stagnant stream flowing from a swamp, on the site of which the Field of Mars is now located.

In 1704, Peter I took on the "inhabited and advantageously located estate [at the source of the Fontanka], where the property of Swedish Major Erich Berndt von Konow (Konau) was located — a small house with a farmyard and garden," with the aim of arranging his summer residence here with an extensive garden — the future Summer Palace and Summer Garden. At the same time, the tsar ordered the first garden builder, Ivan Ugryumov, to "cut through that river" — which his associate accomplished the following year: on the 1705 plan, the Fontanka was already dammed. It is not surprising that the source of the Moika silted up completely, and the nearest surroundings of the Summer Garden turned into a swamp. After this, in 1711, the dam had to be broken through, replacing it with a permanent bridge. In addition to clearing the natural source of the Moika, restoring its natural connection with the Fontanka, the Lebyazhya Ditch was dug along the western edge of the Summer Garden, which connected the Moika directly with the Neva. Thus, on the eastern edge of the historic Usaditsa Island, a new artificial island formed between the Neva, Fontanka, and Moika, officially called the Summer Garden Island. In 1711, the Moika River was connected with the Fontanka (formerly called the Nameless Channel). In 1710, the "Great Perspective Road" (now Nevsky Prospect) was laid on the left bank of the Neva, and at the place where the road crossed the river, a natural boundary of St. Petersburg was formed. A wooden bridge was built here, and at the beginning of the century, transit duties were collected here; for this purpose, the Mytny Dvor was built near the bridge, not far from the Gostiny Dvor.

In the mid-18th century, the first wooden embankments of the Moika River were constructed, and the riverbed was deepened. In the early 19th century, the embankments from the Fontanka to English Avenue and Kryukov Canal were faced with granite. In 1960, a granite embankment was built near the Mikhailovsky Garden, and in 1975 — the embankment from Maklin Avenue to the Pryazhka River.

Now flowing through the central part of the city, the Moika washes the southern side of Admiralty Island (the part called the 2nd Admiralty Island); it flows into the Neva (the Great Neva) from the left. From the Moika branch off the Winter Ditch and Lebyazhya Ditch, connecting it with the Neva, as well as the Griboedov Canal, which flows into the Fontanka.

Sources:

https://ru.wikipedia.org/wiki/Moika_(river)

https://journal.sovcombank.ru/puteshestviya/reka-moika-v-sankt-peterburge-istoriya-nazvaniya-i-dostoprimechatelnosti