Either connected to other water courses, even in the distant past, or human intervention. No such thing as eggs attached to bird’s legs surviving the desiccating flight.

But I do believe in magic!
References :
Ex-fisheries biologist…

There’s only four ways that fish can get into another water body: floods; by ‘waking’ over land; transportation by another species; and in extremely rare circumstances, through tornados that siphon water.

Floods happen regularly and they are the most common mechanism for temporarily connecting water bodies. Not many fish can travel very far out of water – no species native to North America travel across land for more than a few feet.

Its pretty rare for an animal other than humans to transport fish. Its possible than osprey might drop a fish, or that an egg mass could get stuck to the foot of a wading bird and get deposited alive somewhere else, but these types of things have an extremely low probability of happening. Only those fish species that have sticky eggs are likely to be moved by birds. There are no records of fish introductions by birds, but the possibility has not been disproved.

Tornados occcasionally cross water bodies and siphon small fish and dump the fish elsewhere, but this happens in the US only a few times per century, and the fish don’t usually travel very far from their source. There are a few documented cases of fish showing up on the ground following tornados, but it is no known if this mechanism has ever introduced fish to a new water body.
References :

Such areas soon become populated with aquatic plants and animals.
Then they attract many kinds of wildlife that come there to drink,
bathe, prey and feed, or build their homes. Such areas have life, beauty
and interest the whole year round. We call them "Wildlife Oases". But
they present one problem, important in a county of 4,500,000 people:
mosquitoes.

Fortunately, the Chicago region has a fish immigrant from the
southern state with which we control the mosquitoes that breed in such
waters. It is the Mosquitofish, or Gambusia, one of the little
topminnows or killifishes, and a near relative of the guppies,
swordtails and moons — popular aquarium fishes also from warmer
climates. Like them, and unlike the other native killifishes of the
northern states, the young of the Gambusia are born alive. It is called
the mosquitofish because, more than any other kind, it regularly feeds
among trash and vegetation in shallow water and along shores where
mosquitoes breed.

It was first successfully introduced into northern Illinois in 1923, when
some of these fish were brought from a pond on the campus of
Southern Illinois Normal University, at Carbondale, and placed in a
garden pool in Winnetka, a north shore suburb of Chicago.
Carbondale, over 300 miles south, is near the northern limit of the
Gambusia’s normal range.

In 1928 and 1929, more of these little fish from the Carbondale pond
were placed in ponds on golf courses near Chicago by the DesPlaines
Valley Mosquito Abatement District; but none survived in a winter.
So, in 1933, this organization obtained mosquitofish from the
Winnetka pool and placed them in ponds in our forest preserves and
elsewhere. Enough of these survived and multiplied so that these
ponds have served as hatcheries for further distribution of this hardy
"naturalized" northern strain, obtained by unique good fortune from
one or more rare individual fish adapted to survive long winters
beneath the ice. Since 1941, some of this same strain have been
successfully planted in a variety of Michigan waters as far north as the
Straits of Mackinac.

The two sexes of the mosquitofish are more strikingly different in size
than any other native fish. The mature female is usually less than two
inches long but she is twice as long and about ten times as heavy as
the mature male. Females usually produce 3 or 4 broods in a season
and an average of about 50 young per brood, but exceptionally large
females may give birth to broods of 300 young. Apparently,
mosquitofish do not often live longer than two years.

Mosquitoes lay little raft-like masses of eggs on the surface of water.
These hatch out larvae which must come frequently to the surface to
breathe. After they become pupae, the pupa dangles from the surface
film of the water by a tube thru which it breathes. Mosquitofish have
upturned mouths, and they work along the surface, gobbling down
mosquito eggs, larvae and pupae. In some waters these fish multiply
until they not only effectively control mosquitoes but also serve as an
important item of food for hook-and-line fish. It may develop that the
introduction of Gambusia into waters of the "vacation regions" of the
northern states may accomplish more, in two ways, to increase the
pleasure of vacationists than many more expensive programs of fish
management and of mosquitofish control.

Water plus Gambusia equals recreation minus mosquitoes.
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