INCOMPETENT TRUNKS AND TRIBUTARIES THAT DON'T PLAY FAIR: GRAVEL DELTAS AT TRIBUTARY JUNCTIONS IN CENTRAL NEW YORK

In the Appalachian plateau of upstate New York, we find numerous gravel deltas at tributary junctions clogging the main rivers draining this region. These gravel deltas have not been described in the literature. Here, we provide basic information on the size, shape, and grain size characteristics of these deltas. We floated roughly 100 km of stream length, and documented 30-40 gravel deltas. The presence of these deltas implies significant incompetence of the main stem channels in this area. Why would the tributaries and main trunks be so far from equilibrium from each other? To answer this question completely requires knowledge of the age of the deltas. Are they modern and a product of the flood of record in 2006? Are they historic and representative of human-induced changes in hydrology due to road runoff or land use? Do they span the Holocene? To discriminate between these hypotheses we looked for signs of age on the deltas. Mature trees occupy the higher portions of many of the deltas, ruling out the likelihood that these all resulted from the 2006 flood event. We are still unsure of whether these deltas are historic, or were active through the Holocene. The region was covered by ice sheets, and pre-existing drainages of the Appalachian plateau were somewhat modified by glacial erosion, and backfilled with fluvio-glacial sediment during deglaciation. Trunk streams in the area are alluvial, while many of the lower order tributaries cut bedrock (Devonian sandstone and shale). Many of the tributaries show up on USGS topographic maps as narrow ravines cut into bedrock, suggestive of longer term erosion. Thus, we favor a Holocene age range for these deltas. The plot thickens. Many tributaries do not have deltas at junctions with the main channels, yet still show up as ravines on topographic maps. We measured drainage basin area, channel gradient and stream profile, grain size characteristics, and delta size in order to address the puzzling variation in delta occurrence. We find that delta size and median grain size scale with drainage area and stream power. Non-delta tributaries are not transporting gravel to the trunks, either because of alluvial fan deposition, modern reservoirs, or some other unknown reason.