Detailing on choropleth maps

In epidemiology and public health we often need to represent health problems using rates rather than individuals. Clearly when mapping rates (ill/population) we can’t use spot maps (e.g. John Snow’s cholera map)but need instead to resort to choropleth (area) maps. Examples cancer maps are in ET’s first book. The problem with choropleth maps is that the shading would tend to obscure or obliterate any other geographic features. Accordingly, the value of the map as a layered instument for analysis, comparisons, relationships etc. is lost. It seems to me that it would be possible given a careful choice of colors or shadings to add additional layers to the map showing geographic (rivers, roads, high altitude) or environmental features that might be related to the disease distribution. However, I can find no examples (except for political boundaries or an ocasional superimposed spot) even in cartography texts. Do you know of any example choropleth maps showing other critical geographic features? — any suggestions on shading or coloring?

Thanks,

Bob Fontaine