You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Genghis_Khan

Context:
In 1226, immediately after returning from the west, Genghis Khan began a retaliatory attack on the Tanguts. His armies quickly took Heisui, Ganzhou, and Suzhou (not the Suzhou in Jiangsu province), and in the autumn he took Xiliang-fu[disambiguation needed]. One of the Tangut generals challenged the Mongols to a battle near Helan Mountains but was defeated. In November, Genghis laid siege to the Tangut city Lingzhou and crossed the Yellow River, defeating the Tangut relief army. According to legend, it was here that Genghis Khan reportedly saw a line of five stars arranged in the sky and interpreted it as an omen of his victory.

Question:
What river did Genghis Khan cross before defeating the main Tangut army?

the Yellow River