On May 8, 1846, Zachary Taylor and 2,400 troops arrived to relieve the fort. However, General Arista rushed north with a force of 3,400 and intercepted him about 5 miles (8 km) north of the Rio Grande River, near modern-day Brownsville, Texas. The U.S. Army employed "flying artillery", their term for horse artillery, a mobile light artillery mounted on horse carriages with the entire crew riding horses into battle. The fast-firing artillery and highly mobile fire support had a devastating effect on the Mexican army. In contrast to the "flying artillery" of the Americans, the Mexican cannons at the Battle of Palo Alto had lower-quality gunpowder that fired at velocities slow enough to make it possible for American soldiers to dodge artillery rounds. The Mexicans replied with cavalry skirmishes and their own artillery. The U.S. flying artillery somewhat demoralized the Mexican side, and seeking terrain more to their advantage, the Mexicans retreated to the far side of a dry riverbed (resaca) during the night and prepared for the next battle. It provided a natural fortification, but during the retreat, Mexican troops were scattered, making communication difficult.