Hey,
The error is related to a variable that is getting very big like beyond a limit.
If you've activated gwflow, try using very low streambed hydraulic conductivity (something like 1.0e-07). My experience for this error so far tells me that there might be something wrong with the aquifer zones and the values assigned to streambed hydraulic conductivity and aquifer hydraulic conductivity. One may want to use a more detailed permeability map with more realistic aquifer properties.
Cheers,
Ehsan