Where are smelts found, and why are most freshwater smelts not widely distributed inland?

Smelts (family Osmeridae) are small, silvery fishes found in the cold northern waters of Europe, Asia, and North America. While they are primarily marine or anadromous—meaning they live in the sea and ascend rivers to spawn—some species are permanent freshwater inhabitants.123

Most freshwater smelts are not widely distributed inland because their presence in these environments is associated with relatively recent geological events. Unlike salmonids, which have utilized anadromous life cycles and marine invasion routes to expand their range into newly habitable, glaciated regions, no smelt species has penetrated far enough inland to establish a broad, widespread distribution in fresh water. Their limited inland range stands in contrast to the salmonid fishes, which are the dominant species in many of the Northern Hemisphere's recently glaciated lakes and streams.2