In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: There are more than 20 major dams on the Willamette's tributaries, as well as a complex series of levees, dikes, and channels to control the river's flow.
The only dam on the Willamette's main stem is the Willamette Falls Dam, a low weir-type structure at Willamette Falls that diverts water into the headraces of the adjacent mills and a power plant.  The locks at Willamette Falls were completed in 1873.  Elsewhere on the main stem, numerous minor flow-regulation structures force the river into a narrower and deeper channel to facilitate navigation and flood control.The dams on the Willamette's major tributaries are primarily large flood-control, water storage, and power-generating dams. Thirteen of these dams were built from the 1940s through the 1960s to be operated by the United States Army Corps of Engineers (USACE), and 11 of those produce hydropower. Flood-control dams operated by the USACE are estimated to hold up to 27 percent of the Willamette's runoff. They are used to regulate river flows so as to cut peaks off floods and increase low flows in late summer and autumn, and to divert water into deeper, narrower channels to prevent flooding. In addition, a relatively small of amount of the water stored in the reservoirs is used for irrigation.
What stores a small amount of water for irrigation?