What river does The Transcontinental Railroad cross 31 times?

From: "Charlie Kirk"

What river does The Transcontinental Railroad cross 31 times?

This was referred to in a The Transcontinental Railroad Almanac:

It took six years and two armies totaling 20,000 men. Many of the workers were immigrants from China and Ireland who sweated long hours for one or two dollars a day. They laid tracks across hundreds of miles of prairie and scorching desert. They pushed over heights of 8,000 feet and tunneled their way through hard mountain ridges, sometimes at a rate of only a few inches per day. They bridged stream after stream. The tracks crossed one river alone thirty-one times. ...

—Charlie Kirk