The fact that I prefer fishing from shore has nudged me toward rivers—and that, in turn, led to an unexpected realization: rivers seem to hold an extraordinary diversity of fish. That observation made me wonder whether rivers actually harbor more species than lakes. It turns out they do—and it’s not even close.

So why is that? The short explanation is simple: rivers create more ecological niches per mile than lakes, and niches create species. This reflects a broader ecological principle. Ichthyologists have shown that lotic systems (flowing water) tend to support greater species richness at local scales than lentic systems (still water).

Again, why is that? Rivers generate a remarkable range of habitats—riffles, runs, pools, backwaters, floodplains, estuarine transitions, springs, tributaries, and headwaters. Each of these environments supports different species. In addition, rivers contain fine-scale microhabitats (eddies, seams, root wads, undercut banks) that further expand ecological opportunities. Lakes simply lack this degree of structural complexity.

Rivers also function as connected networks spanning large geographic areas. This allows for dispersal, isolation, recolonization, and ultimately speciation and helps explain why entire groups—darters, shiners, redhorse, and suckers—are predominantly riverine and have diversified into dozens (or hundreds) of species. Lakes, in contrast, are more like isolated bowls: excellent for stability, but less conducive to generating new species.

This pattern is especially evident in the southeastern United States. River systems such as the Tennessee, Cumberland, Mobile, and Apalachicola basins contain the highest freshwater fish diversity in North America—and among the highest in the world outside the Amazon. These are river systems, not lake systems.

So how large is the difference? Of the roughly 800+ freshwater fish species in the United States, the vast majority are associated with rivers and streams. Many are endemic to single watersheds. True lake-specialist species are relatively rare (for example, some ciscoes and pupfish), numbering only a few dozen. By contrast, river-dominated groups are extraordinarily diverse: redhorse (15+ species), darters (200+), minnows (300+), suckers (70+), and madtoms (30+).

None of this diminishes the importance of lakes—they excel in other ways. Lakes support high biomass, offer stable environments, and sustain large, iconic predators such as walleye, pike, and lake trout. They are also central to recreational fisheries. But they are not the primary engines of speciation.

If your goal is species diversity, rivers and streams are the heart of freshwater biodiversity in the United States. Lakes are wonderful—but rivers are where evolution does some of its most creative work.