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Wildlife Corridors

Upper Arroyo Seco, Cottonwood Canyon and Hahamongna Wildlife Corridors Survey and Enhancement Opportunities

Michael Long, Biologist, Arroyos & Foothills Conservancy Advisor

Survey Location:

A walking survey of the upper Arroyo Seco, just south of Devil’s Gate Dam, was conducted August 17, 2013, 8:30 a.m. – 12:30 p.m. (John Howell, Andy Lichucki, Michael Long, Jim Osterling, and E.J. Remson) for the purpose of locating probable wildlife movement corridors linking the San Rafael Hills, the Arroyo Seco and Hahamongna Watershed Park (a Los Angeles County Significant Ecological Area) (“Hahamongna”).  Each of these three areas is important wildlife habitat:

San Rafael Hills. The San Rafael Hills contain extensive natural open space habitat for animals. The extent of the area is sufficient to satisfy food and shelter needs, daily movement and seasonal migration for wildlife similar to that in the San Gabriel Mountains.

Arroyo Seco South of Cottonwood Canyon. South of the confluence of Cottonwood Canyon with the Arroyo, the Arroyo Seco stretches all the way through Debs Park and on to the Los Angeles River, with the understanding that it constitutes a wildlife corridor (Jeff Chapman, Audubon California at Debs Park).

Arroyo Seco North of Cottonwood Canyon.  Going north from Cottonwood Canyon, the Arroyo Seco is crossed overhead by the 210 Freeway, then is interrupted by Devil’s Gate Dam, which is the southern boundary of Hahamongna.  The Arroyo Seco continues northward, connecting, along with its Millard Canyon and El Prieto Canyon tributaries, with the Angeles National Forest.  Studies have identified this as important wildlife habitat (Stephenson, J.R. & G.M. Calcarone.  1999).

Cottonwood Canyon connects the San Rafael habitat with those of the Arroyo Seco and Hahamongna.

The group walked the upper Arroyo Seco west side trail from near the intersection of Parkview Avenue and West Drive, north past the confluence with Cottonwood Canyon, and continued under the heavily-vegetated 210 Freeway bridge to and over Devil’s Gate Dam and the Flint Wash Bridge. 

Much of the trail route is shaded by oaks and other trees.  It is bounded on the west by a chaparral-covered slope that forms the west bank of the Arroyo and on the east by a band of growth and fencing that is the west boundary of Brookside Golf Course No. 2 (Figures 1 and 2).

In addition to the group’s observations, our analysis relied upon published reports on biological values, corridors and habitat restoration concepts for the Arroyo Seco and vicinity.

Determinations:

This Arroyo Seco route provides an unimpeded wildlife corridor for animals moving north and south of Cottonwood Canyon to Hahamongna and the San Gabriel Mountains.  Cottonwood Canyon, sloping from west to east before intersecting with the Arroyo’s west bank, has already been shown to be outstanding wildlife habitat and a corridor between the Arroyo and the substantial reserve of open space in the San Rafael Hills (Juhasz 2013, Long 2013).

While not physically surveyed, it is apparent that between Linda Vista Avenue to the east and Normandy Drive to the west there is another wildlife route that runs north from Cottonwood Canyon, parallel to the Arroyo corridor (Figures 1 and 2) (the “Normandy-Linda Vista Corridor”).

Figure 1

Figure 2

There are at least three clear and open wildlife movement corridors from the Arroyo Seco and Normandy-Linda Vista corridors north of Cottonwood Canyon that go up and over Devil’s Gate dam into the Hahamongna basin.  While animals likely climb up and over embankments, these routes are facilitated by two existing, large tunnels under Oak Grove Drive immediately south of the dam.  Once on the north end of either tunnel, wildlife can reach Hahamongna by crossing the narrow La Canada-Verdugo Road, which both accesses and crosses over the top of the dam. The road is closed to public traffic and rarely used by vehicles, and is vegetated on both sides.  The Flint Wash riparian corridor leads directly into Hahamongna under bridges just west of the dam.  Accordingly, Hahamongna is a transition habitat and movement corridor between the Arroyo Seco south of the 210 Freeway and the Upper Arroyo Seco, and beyond to the Angeles National Forest.

The Arroyo corridor extends south from Cottonwood Canyon as well, with natural cover of shrubs and trees widening through the low, un-channeled portions of the Arroyo stream, on to Lower Arroyo Park, then through South Pasadena and beyond (see Figure 3, below). The corridor is enhanced by cover on both east and west slopes of the Arroyo.

Along the route we hiked were tracks of coyote, mule deer and California ground squirrel, mingled with human and domestic dog prints.  A sub-adult bobcat was seen bounding uphill from the stream bed under the east side of the 210 Freeway bridge at 11:40 A.M.  A mule deer was observed two weeks prior moving across Linda Vista then downslope, in the vicinity of Cottonwood Canyon (J. Howell, personal observation).

Mammals of the area expected to be using these corridors include mule deer, mountain lion, bobcat, coyote, gray fox, black bears, raccoon, striped skunk, Virginia opossum, long-tailed weasel, Audubon’s cottontail rabbits, California ground squirrel, fox squirrel, western gray squirrel, Merriam’s chipmunk and numerous rodents.  Birder Long was impressed by the quality of birding habitat, for both year-round living space and migration.  In addition, numerous reptiles and amphibians are expected to share the habitat.

Figure 3, below, shows more of the corridors in the San Rafael Hills.

Figure 3

Opportunities for Corridor Enhancement:

Through Devil’s Gate Dam.  While this study demonstrates that wildlife can move through the Arroyo Seco both to and from the Hahamongna basin and Angeles National Forest, physical improvements could enhance wildlife passage, especially where bottlenecks exist.  Both tunnels described above are accessible to humans; an added underpass, tunnel or large culvert dedicated for animals-only under La Canada Verdugo Road could encourage passage without any road crossing.  One possibility would be to open an existing horizontal tunnel that runs under the dam by removing grates to daylight the tunnel. Since this might be a ready solution, the feasibility should be investigated.  Proposals and support for restoration concepts are detailed in studies of the Arroyo Seco by the U.S. Army Corps of Engineers (2011), the Arroyo Seco Foundation (Camp Dresser & McKee 2010) the National Park Service (2012) and Northeast Trees (2006). 

South to Debs Park.  The Arroyo Seco provides a functioning wildlife linkage between Angeles National Forest and Debs Park in Los Angeles (Jeff Chapman, Audubon California at Debs Park). In some sections the linkage is highly functional because of the presence of cover, water and food for animals living within or moving through the wildlife corridor.  These areas include the natural stream section under the 134 Freeway and Colorado Boulevard bridges, the restored wetland and riparian areas in the Pasadena Lower Arroyo Seco Park, and smaller areas containing native plants along the channel to the south. Other areas of the Arroyo Seco are less conducive for wildlife movement but could be improved with habitat restoration or simple land management changes.

Cottonwood Canyon.  Cottonwood Canyon itself, although relatively untouched, can be enhanced to promote wildlife passage, more riparian habitat and improved overall habitat.  See the companion section entitled Biological and Ecological Values of Cottonwood Canyon Property and Enhancement Opportunities.

Conclusion:

Cottonwood Canyon, with its perennial water flow, lush native vegetation and native tree cover, is one of the highly functional tributaries of the Arroyo Seco.  Like the others, it serves as a sort of habitat “stepping stone” for animals moving along the Arroyo.  Eventually it is hoped that these stepping stones will become a highly functioning continuous corridor from the Angeles National Forest to Debs Park and beyond to the Los Angeles River.

On the other hand, development of Cottonwood Canyon would disrupt or even entirely eliminate the wildlife corridor through Cottonwood Canyon, biologically isolating the San Rafael Hills.  Development could negatively impact the spring and jeopardize the year-round flow of water, which could be a major blow to wildlife since they are drawn to it and depend upon it, and there may not be more than one other spring in the San Rafael Hills. It would eliminate many acres of native habitat along and adjacent to the Arroyo Seco, and preclude the opportunity to enhance habitat for plants and wildlife.

Acknowledgements:

This survey and report were compiled by the field team from the Arroyos & Foothills Conservancy identified in the first paragraph and the preparation and review group comprised of that same group plus Tim Brick, Frank Simpson and cartographer John Wros.