Draws line on policies


    The so-called “blue-line” stream is a mapping device used by cartographers to indicate a channel in which water flows. A good example is that of the U.S. Geological Survey 7.5-minute quadrangles of the southern slopes of the Santa Monica Mountains in Malibu. Those maps show, roughly, an average of about 2.5 miles of the blue-line streams per square mile. The Survey defines such streams essentially subjectively based upon aerial photographs showing channel vegetation or perhaps limited field examination. None is based upon detailed hydrologic study of the long-term flow regime nor any kind of environmental data. Nevertheless, the blue-line stream commonly is accorded special status by public agencies concerned with planning, building and safety, health, and the environment.

    Streams are classified, generally, as perennial, intermittent or ephemeral. Perennial streams are those fed by springs or snow melt to such an extent that they flow constantly except during periods of severe drought. Intermittent streams are those that flow only during part of the year but for longer periods than during rains storms or shortly thereafter. Ephemeral streams flow only during storms or shortly thereafter. Beyond this, there are channels in which flow never occurs or occurs only during storms. Such streams might be termed “sub-ephemeral.” Most channels in the Santa Monica Mountains have only sub-ephemeral flows, and many blue-line streams, or sections of them, are either ephemeral or sub-ephemeral.

    The only perennial stream in the Santa Monica Mountains is Malibu Creek, which flows in Triunfo Canyon, and its perennial character may be due simply to effluent from the Tapia sewage treatment facility. Streams in all the other large canyons in Malibu-Topanga Canyon, Tuna Canyon, Las Flores Canyon, Corral Canyon, Trancas Canyon, Arroyo Sequit, Sycamore Canyon and Big Sycamore Canyon-are all intermittent and their many miles of blue-lined tributaries are either ephemeral or sub-ephemeral.

    Ephemeral or sub-ephemeral blue-lined streams in named smaller canyons in Malibu include those in Pena Canyon, Piedra Gorda Canyon, Winter Canyon, Marie Canyon, Puerco Canyon, Dry Canyon, Latigo Canyon, Escondido Canyon, Ramirez Canyon, Walnut Canyon, Encinal Canyon, Lechusa Canyon, San Nicholas Canyon, Deer Canyon and La Jolla Canyon.

    It therefore is reasonable to regard blue-lining of streams in the western Santa Monica Mountains, without some kind of supporting data, as hydrologically and environmentally meaningless. As a consequence, arbitrary departmental rules or policies relating to development near blue-line streams cannot be rationally applied.

    E. D. Michael