LINKING FLUVIAL PROCESSES TO ALLUVIAL STRATIGRAPHY IN OTSEGO COUNTY, NEW YORK

Our project investigates the relationship between alluvial stratigraphy and fluvial processes in in the upper Susquehanna River basin in New York. The area is ideal to study floodplain stratigraphy and development. Valleys were backfilled and re-graded by outwash during deglaciation. Modern rivers gently etch glacial era deposits and landforms. Holocene base level change and tectonic activity are negligible. Two reach types are common: sinuous low gradient rectangular channels with high mud banks; and bar-riffle-pool reaches with steeper gradients and lower banks composed of mixed gravel and fines. Channel bifurcations are common in the latter. Without much drive to incise, trunk streams tend to wander laterally, leaving behind a record written in mud, gravel, and biogenic (tree) material. Gravel transport through the river system appears limited to local riffle-pool reaches, as gravel deltas at tributary junctions are radially symmetric with crisp angle of repose margins. Mud deposits commonly drape gravel bars, form point bars, or settle as lenses in the lee of large woody debris.

Floodplains store a longer term record of channel activity. Locally, cutbanks expose extensive massive buff fine grained deposits beside wide channels with low gradients and 1- 3 m high banks. In bar-riffle-pool reaches, banks reveal gravel bodies with undulating contacts that rise 1-2 m above the bed. The gravel units are often draped with gently dipping mud layers and woody debris lenses, and capped by a massive fine grained layer, presumably overbank fines. We find numerous cases where mud layers drape a gravel bar. Basal floodplain ages are strongly diachronous, ranging from late 20^th C (contains a golf ball) to ~13300 Cal. years BP (¹⁴C).

We observe deposits very similar to legacy mill pond stratigraphy in the mid-Atlantic region of the US. That model attributes dipping mud layers to mill ponds, and basal gravel-gley layers as a hydric soil representative of a pre-colonial floodplain. We are hesitant to follow these interpretations in our area. We think the alluvial stratigraphy of diverse age exposed in central New York cutbanks faithfully reflects current channel processes—a result which suggests that the past looks akin to the present. To attribute the above features to mill pond sedimentation will require careful detailed dating.