Slight coloration differences (lighter/darker, speckled or not) or even slightly difference in overall shape of certain individuals from different locations does not mean they are different species.
The topography of one river, compared to the next, can influence which colors or shapes are more survivable in one vs. the other, only a mile or two down the road.
If the substrate is dark and stoney in a certain river, the majority of a spawn that is less easily predated upon might be darker, more mottled, and pass on the genes to favor those dark individuals.
Same thing might occur if the substrate is sandy, and favor those of a spawn with lighter less mottled coloration, to survive by having a lighter overall background.
Other factors such as overhanging terrestrial vegetation, and how much shade is created, or whether flow makes the surface of the water sparkle, may also influence what individuals are able to pass on genes.
One of the favorite examples of a species I kept, is Chuco intermedia.
I kept variants from different two separate rivers, each river had different topography, and the variation within the species was striking.
Although both are the same species, color, and even spotting patterns are obviously quite different.
Of course one of the most obvious examples of this, is Vieja melanurum, and synspillus, where for years they were thought to be different species by experts, because of their striking color differences, but.....
we now know, thru DNA sequencing, they are simply color variants of the same species, V. melanurus, each adapted to different habitat topography.