gistfile1.txt

/* River lines */

.river::line { color: #acf; }

/* I am guessing as to the meaning of the CLASS column. 
 * This style appears mostly correct */

.river::line[zoom=7][CLASS=1]  { width: 0.1; }
.river::line[zoom=8][CLASS=1]  { width: 0.2; }
.river::line[zoom=9][CLASS=1]  { width: 0.4; }
.river::line[zoom=10][CLASS=1] { width: 0.8; }
.river::line[zoom=11][CLASS=1] { width: 1.6; }
.river::line[zoom=12][CLASS=1] { width: 2.4; }

.river::line[zoom=8][CLASS=2]  { width: 0.1; }
.river::line[zoom=9][CLASS=2]  { width: 0.2; }
.river::line[zoom=10][CLASS=2] { width: 0.4; }
.river::line[zoom=11][CLASS=2] { width: 0.8; }
.river::line[zoom=12][CLASS=2] { width: 1.6; }

.river::line[zoom=9][CLASS=3]  { width: 0.1; }
.river::line[zoom=10][CLASS=3] { width: 0.2; }
.river::line[zoom=11][CLASS=3] { width: 0.4; }
.river::line[zoom=12][CLASS=3] { width: 0.8; }