Stream lines shading should be scaled to avoid barely visible stream lines

The colour of the vector field lines in stream is set to depend on the vector field magnitude. However, in some cases, (see Epidemic Models Demo notebook stream 2 (§1.3) and attached image), the resulting stream plot is barely visible because all stream lines have a light grey colour. Is it possible to scale the color gradient according to the max magnitude in the current plot such that the stream lines are always clearly visible?