Isopleths

An isopleth is a contour line of equal value, much like the images seen below.  These can be used to represent anything to signify constant value.  Here the map is showing the various levels of chlorides in the river.

http://content.cdlib.org/data/13030/r5/kt4j49n6r5/figures/caljsiol_sio1ca175_133_042.gif