Stream Channel Development
|This geomorphic profile shows interpretation of the relative ages and character of Quaternary alluvial channels and terrace deposits in the Mojave region (Dave Miller, USGS; to see a larger version of this image, click here). Note that no vertical elevation is applied. Typically the greatest elevation differences between active channels and older terrace deposit occur high on an alluvial fan and may range measurably by tens of meters. Low on alluvial fan the differences between oldest and youngest deposits may be less than a meter.|
Qya1 - "Quaternary young alluvium" (Qya); the "Qya1" represents an active stream channel and its deposits; the channels tend to be void of vegetation and display evidence of recent stream flow activity.Qya2 - represents older flood plain surfaces that have not been disturbed by flooding long enough for plants to become established.Qya2 surfaces typically have the greatest biomass of all surfaces. This is probably a combination factors: roots can take advantage of the loosely consolidated deposits and access to moisture at depth. As roots stabilize the Qya2 surface flood overbank fines can allow for increased moisture retention. Qya3 & Qya4 - represents mid- to older-Holocene alluvial fan surficial deposits that are elevated and isolated from active channel erosion. These surfaces have a thin, intermittent soil, often with accumulation of floodplain overbank deposits and wind-blown silt deposits on surfaces that have not experienced significant erosion. These surface are generally sparsely vegetated relative to Qya2 surfaces.
Qia1, Qai2, & Qia3 - represent progressively-older, late Pleistocene age deposits and surfaces that range in age from about 20,000 to 400,000 years. These deposits and surfaces possibly correspond to cycles of aggradation and erosion in conjunction with alternating wet and dry climatic cycles in the Mojave region during the late Pleistocene. Older surfaces of Qia type tend to be barren of plants relative to the younger Qya surfaces, and may have well-developed desert pavement that may cap a thin silty, loam soil (vesicular A horizon) that overlies a more weathered, dense clay-rich subsoil (Argillic B horizon). Qai1 to Qai3 surfaces tend to be best preserved in the middle portion of "stable or well-developed" alluvial fans, or occur as elevated bench-like terraces along stream channels with broad valleys.
Qoa - represents middle Pleistocene and channel deposits and terraces that are roughly older than 400,000 years. These deposits are typically highly eroded, and are locally represented at rounded hills consisting of tightly cemented (caliche) alluvial gravels or paleosols (ancient soils horizons), typically along the range front of mountains. In Qoa deposits, the sediment's cement is dominantly calcium carbonate precipitated from meteoric (atmospheric water) and groundwater in the past, but the process is ongoing. In desert conditions, infrequent rains dissolve calcium carbonate from the upper soil, and carries dissolved components downward. The accumulation of calcium carbonate through plant transpiration processes creates an impenetrable caliche (calcic horizon) in the lower subsoil. In some places, the calcium carbonate and other mineral precipitates may have been contributed by migrating groundwater. Older Pleistocene and late Tertiary deposits have long since been eroded away in the Mojave region, or are locally incorporated in older basin-fill deposits; there is generally no surface expression remaining for these older deposits/features.
Stream Channel Processes
Even in the mountainous regions most streams flow only during or shortly after storms. Perennial water only flows in groundwater discharge areas associated with springs in a few mountain canyons, in Afton Canyon where the regional groundwater table intersects the canyon bottom, and a few other springs. In most areas within the Mojave region, streams will flow only after long periods of steady rain, typically during a wet winter. The periodicity and intensity of such rain events depends on elevation, but in the lower regions historically floods may only happen in intervals measured in several years to decades. To see the history of flood frequency in the Mojave region over the 20th Century see the on-line report by Hereford and Webb, (2000).
Floods produce the visually definable channels in streambeds (active channels). When water is not flowing in the stream between storm events, an active channel typically consists of sand, gravel, dried mud, or barren bedrock. Cut and fill sediment bedforms appear relatively fresh (where not trampled by animals, including humans). Flowing water strips away vegetation, moves sediment, and reconfigures bedforms in the channel. Sediment character and supply, slope, and flow volume and duration are controlling factors that defines the size of stream channels and the character of sediment found in the barren channel once a flood event is over. In canyons above the mountain front, stream channels are typically filled with angular rock fragments ranging from coarse sand to great boulders, with rapids or falls occurring where bedrock is exposed in the stream channel. Larger floods can scour the channel clear of sediments, whereas lesser flood events can contribute to the backfilling of channels. Backfilling is most evident to desert travelers who frequently travel the same stream beds year after year. In one year a stream bed in mountainous area may be easily passable by vehicle, but the next year the wash is inaccessible because finer materials between larger boulders may have vanished due to an erosion event. Later, the fine deposits between boulder may reaccumulate after a different storm event. These changes reflect the differences in duration, spatial patterns, and intensity of individual storm events affecting a drainage basin.
Downstream of a mountain front streams deposit sediments on alluvial fans, and in in the more upland areas, the channels on the upper alluvial fan may go through periods of down cutting, infilling, and channel migration. Typically the size and depth of the channel, and the size of the rock fragments diminish in size down slope and away from the mountain front. In the mid to lower fan area, stream channels typically diminish to depths less than a meter, and sediment consists of fine gravel and sand. In most areas, a trunk stream defines the main drainage between coalescing alluvial fans, or playas (dry lake beds) may exist were topographic barriers impede the flow of surface water from a drainage basin.
Examples of Stream Channel Features
|This image shows is a view of an upper fan area in the western Providence Mountains. The "active channel" on the left (Qya1), and another less active, higher channel on the right that only receives water during the most intense flooding events (Qya2). Mature, or well-established, vegetation populates parts of the flood plain that generally does not receive flash flood waters (Qya3 and Qya4). The yellowish plant is cheesebush, a plant adapted to rapid colonization of disturbed surfaces and common in washes.|
|This view looking west from the mountain front of the Providence Mountains looking down a wash that has incised into older (Pleistocene) alluvial fan deposits. The wash continues down slope and eventually merges with the actively aggrading surface of the lower fan closer to the axial trunk stream (Kelso Wash) that drains the fan aprons draining from the Providence Range and the Kelso Mountains. Note the diverse plant community on the high surface in the foreground.|
|Cedar Wash is the main channel draining the northern Providence Mountains region. The break in slope along the Holocene stream valley is marked by a vegetation change between the Joshua-tree forest on the slopes, and the desert scrub-covered sandy gravel of the flood plain. The barren active channels on the modern flood plain stand out as tan lines. Down cutting by Cedar Wash into older Quaternary alluvial fan deposits has created a well-developed terrace along the valley. This down cutting probably occurred during the wetter regional conditions associated with the last glaciation period that ended roughly 15,000 years ago. With the dry conditions that exist today, the valley is gradually filling in with alluvium because the stream can no longer move sediments faster than they accumulate.|
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USGS Western Region Geology and Geophysics Science Center