Instructions: 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.
Input: Passage: Balch Creek begins in the Forest Park neighborhood in unincorporated Multnomah County near the intersection of Northwest Skyline Boulevard and Northwest Thompson Road at the crest of the West Hills. It flows generally east about 3.5 miles (5.6 km) to its confluence with the Willamette River, a major tributary of the Columbia River. The creek drops from 1,116 feet (340 m) above sea level at its source to 46 feet (14 m) at its mouth, a total of 1,070 feet (330 m). Most of this occurs in the first 2.5 miles (4.0 km). In the hills, the stream gradient (slope) generally ranges from 15 to 30 percent interspersed with sections of less than 15 percent along the middle reaches.From its source, the creek runs east on private property near Forest Park, a large municipal park in Portland, before turning briefly south about 3 miles (5 km) from the mouth. Soon thereafter, the stream receives an unnamed tributary on the right bank and turns southeast on private property along Northwest Cornell Road. It enters the city and the Audubon Society of Portland simultaneously about 2 miles (3 km) from the mouth, receives two more unnamed tributaries on the right, and flows northeast, entering the part of Forest Park known as Macleay Park.For about 0.25 miles (0.40 km) the creek parallels Wildwood Trail, the main hiking trail in Forest Park, until reaching a former public restroom known as the Stone House. From here Balch Creek runs beside the Lower Macleay Trail, another hiking trail, for about 0.8 miles (1.3 km). Near Northwest Thurman Street, roughly 1 mile (1.6 km) from the mouth, the creek flows through a trash rack into an 84-inch (210 cm) diameter storm sewer. City workers first diverted the creek into a pipe in the early 20th century. The water empties into the Willamette River in the city's Northwest Industrial neighborhood at Outfall 17. About 10 miles (16 km) below this outfall, the Willamette enters the Columbia River.
Output:
What is a tributary of for the Willamette River?