Question: What is the mouth of the river which serves as the mouth of the Bumping River?

Knowledge 1: Bumping River
The Bumping River is a tributary of the Naches River, in Washington in the United States. It flows down the east side of the Cascade Range, through Wenatchee National Forest and the William O. Douglas Wilderness. From its source at Fish Lake near Crag Mountain, it flows northeast to Bumping Lake, a natural lake enlarged and regulated by Bumping Lake Dam. Below the dam, the Bumping River continues flowing northeast. It is joined by the American River, its main tributary, a few miles above its mouth where it joins the Little Naches River to form the Naches River. [SEP] Knowledge 2: Naches River
The Naches River is a tributary of the Yakima River in central Washington in the United States. Beginning as the Little Naches River, it is about 75 miles (121 km) long. After the confluence of the Little Naches and Bumping River the name becomes simply the Naches River. The Naches and its tributaries drain a portion of the eastern side of the Cascade Range, east of Mount Rainier and northeast of Mount Adams. In terms of discharge, the Naches River is the largest tributary of the Yakima River.
Knowledge 1: Bumping River
The Bumping River is a tributary of the Naches River, in Washington in the United States. It flows down the east side of the Cascade Range, through Wenatchee National Forest and the William O. Douglas Wilderness. From its source at Fish Lake near Crag Mountain, it flows northeast to Bumping Lake, a natural lake enlarged and regulated by Bumping Lake Dam. Below the dam, the Bumping River continues flowing northeast. It is joined by the American River, its main tributary, a few miles above its mouth where it joins the Little Naches River to form the Naches River. [SEP] Knowledge 2: Plymouth
The settlement of Plympton, further up the River Plym than the current Plymouth, was also an early trading port, but the river silted up in the early 11th century and forced the mariners and merchants to settle at the current day Barbican near the river mouth. At the time this village was called Sutton, meaning south town in Old English. The name Plym Mouth, meaning "mouth of the River Plym" was first mentioned in a Pipe Roll of 1211. The name Plymouth first officially replaced Sutton in a charter of King Henry VI in 1440. See Plympton for the derivation of the name Plym.
Knowledge 1: Bumping River
The Bumping River is a tributary of the Naches River, in Washington in the United States. It flows down the east side of the Cascade Range, through Wenatchee National Forest and the William O. Douglas Wilderness. From its source at Fish Lake near Crag Mountain, it flows northeast to Bumping Lake, a natural lake enlarged and regulated by Bumping Lake Dam. Below the dam, the Bumping River continues flowing northeast. It is joined by the American River, its main tributary, a few miles above its mouth where it joins the Little Naches River to form the Naches River. [SEP] Knowledge 2: Cabramatta Creek
Cabramatta Creek, an urban watercourse of the Georges River catchment, is located in the south-western region of Sydney, in New South Wales, Australia.

Answer: Yakima River