It's
not glycerin. I have no idea why the BB people are telling people this -- it's not correct. If the "rivers" were really glycerin, they'd be sticky and goopy, and despite what the BB people say, they're not.
Rivers are just soap that looks different. This is an esthetic issue that bothers some people and not others. Except for the appearance, the soap is perfectly fine.
"Glycerin" rivers aren't caused by too much water or insulating, speaking strictly. Rivers are caused by the soap getting warm enough to go into gel (a semi-liquid stage in which soap is a sticky semi-translucent paste)
and then cooling slowly. Both factors -- gel and slow cooling -- have to be present to get visible rivers.
Rivers are more obvious when pigmented colorants are used (titanium dioxide is well known for this), but any soap can have rivers -- it's just you might not always see 'em.
The temp at which soap goes into gel is lower if you use more water versus less. I'd say a 33% lye concentration (2:1 water:lye ratio) is a good choice to minimize the chance of rivers. I get them sometimes when I soap at 31% lye concentration, but seldom if I use 33% lye conc.
Insulating can cause the soap to cool slowly enough for the rivers to form if you choose not to reduce the water content. Reduce the chance of rivers by ensuring the soap cools more rapidly after it reaches gel temp. Don't insulate quite so much, or remove the insulation after the soap reaches gel so the mold is in the cooler open air, or cool the soap in front of a fan.
But honestly, the easiest solution to prevent rivers is to use less water so the soap has a higher gelling temperature.
Keep in mind if you use colorants mixed with water that this water will also raise the water content. You can get rivers in the parts of the soap where you add these colorants. It's easy to think "oh, it's just a tiny amount of extra water, it won't cause rivers" but it definitely can.
Soap that gets hot enough to go into gel is physically harder, so you can unmold and cut shortly after saponification is done. If the soap doesn't get that warm, it tends to be softer and sometimes crumbly which makes the soap more difficult to unmold and cut. Colors tend to be more saturated in a gelled soap, and the soap is often more translucent rather than opaque. On the downside, soap with dairy milk tends to be a little darker if it gels.
More info --
https://classicbells.com/soap/streakMottle.asphttps://auntieclaras.com/2018/05/how-to-make-glycerine-rivers/