Alluvial Data Research Articles

Influence of sediment supply, lithology, and wood debris on the distribution of bedrock and alluvial channels

Field surveys in the Willapa River basin, Washington State, indicate that the drainage area‐channel slope threshold describing the distribution of bedrock and alluvial channels is influenced by the underlying lithology and that local variations in sediment supply can overwhelm basinwide trends. Field data from 90 short-reach surveys indicate that about one-eighth of the surveyed reaches do not conform to a threshold defined by data from free-formed alluvial and bedrock reaches due to the effects of logjams or local sediment sources or sinks. Mapping of channel type distributions in 18 extended reconnaissance surveys of >100 channel widths in channel length show that ~75% of the channel network was alluvial, but that the proportion of forced alluvial channels varies from 0% to 84%. Using the drainage area‐slope thresholds defined by bedrock and alluvial data from the short-reach surveys, only 40% of the total channel length mapped in the longer reconnaissance surveys was correctly classified from a 10 m grid digital elevation model. Of the misclassified reaches, 80% of the alluvial channels predicted to be bedrock had forced alluvial morphologies, while almost half of the bedrock channels predicted to be alluvial were forced by low sediment supply, typically due to their location immediately downstream of large channel-spanning logjams. Poor representation of reach-scale slope in the digital topography and/or a stochastic influence of sediment wave propagation likely account for the remaining misclassified channels, which together compose 7% of the total surveyed channel length. Although variations in sediment supply can locally overwhelm the channel type predicted by the threshold model, the effect of logjams masks any influence of propagating sediment waves on the distribution of bedrock and alluvial channels in the Willapa River basin.

Postglacial Timberline Fluctuations, La Plata Mountains, Southwestern Colorado

A paleoecological study of the La Plata Mountains was initiated to develop further a dated and continuous environmental sequence for the San Juan River headwaters to which discontinuous archaeological and alluvial data from the Four Corners region might be compared. Routine pollen analysis of a 4-m core from a subalpine meadow adjacent to Twin Lakes, 3290 m (10,790 ft), including 11 radiocarbon dates and pollen ratios, provides the chronology of climatic change as reflected by postglacial timberline fluctuations. The timberline was lower than that of the present about 9800 BP, then advanced upward at least twice to higher elevations prior to 6000 BP. The timberline retreated to lower elevations shortly after 4000 BP; this retreat was followed by another significant advance upward about 2500 BP. Mining, logging, and grazing, which began in the 1870s, may be represented by a sharp decrease in the relative frequencies of Pinus and Picea pollen, with subsequent secondary succession represented by increased Ranunculaceae and Salix pollen and then a return to conifer pollen dominance. These changes may also result from a significant lowering of the timberline within the last few hundred years.