Geological, hydrological, and biological issues related to the proposed development of a park at the confluence of the Los Angeles River and the Arroyo Seco, Los Angeles County, California

The Mountains Recreation and Conservation Authority (MRCA) is considering plans for developing a new park at the confluence of the Los Angeles River and the Arroyo Seco in Los Angeles County, California (fig. 1, 2). This location is referred to as “the confluence” in the remainder of this report. The Arroyo Seco flows into the Los Angeles River at the southern end of the Glendale Narrows, just before the river enters the coastal plain. As in much of its 51-mi reach, the river here flows through a concrete-lined channel. Approximately 1,000 ft upstream from the confluence, the bottom of the Los Angeles River is unlined. The Arroyo Seco also is lined with concrete in this region.