A self-propelled irrigation system has an elongated pipeline supported above a field by a plurality of drive towers. The drive towers each have a base beam assembly, a tower structure extending above the base beam assembly for supporting the elongated pipeline, and a plurality of drive wheel assemblies for moving the base beam assembly over a field. The drive assemblies include first and second drive assemblies mounted to respective ends of the base beam assembly. At least one of the drive towers also has a third drive assembly mounted to the base beam assembly between the first and second drive assemblies. The base beam assembly supporting the third drive assembly is articulated between its first and second ends and pivotally connected to the lower ends of the tower structure to allow the third drive assembly to move vertically relative to the first and second drive assemblies when traversing uneven terrain.