No abstract

Geomorphic mapping in the upper Conejos River Valley of the San Juan Mountains has shown that three distinct periods of aggradation have occurred since the end of the last glacial maximum (LGM). The first occurred during the Pleistocene–Holocene transition (~ 12.5–9.5 ka) and is interpreted as paraglacial landscape response to deglaciation after the LGM. Evidence of the second period of aggradation is limited but indicates a small pulse of sedimentation at ~ 5.5 ka. A third, more broadly identifiable period of sedimentation occurred in the late Holocene (~ 2.2–1 ka). The latest two periods of aggradation are concurrent with increases in the frequency of climate change in the region suggesting that Holocene alpine and sub-alpine landscapes respond more to rapid changes in climate than to large singular climatic swings. Soil development and radiocarbon dating indicate that hillslopes were stable during the Holocene even while aggradation was occurring in valley bottoms. Thus, we can conclude that erosion does not occur equally throughout the landscape but is focused upslope of headwater streams, along tributary channels, or on ridge tops. This is in contrast to some models which assume equal erosion in headwater basins.

Pollen assemblages, diatom assemblages, and sedimentology, from Cumbres Bog in the southeastern San Juan Mountains of Colorado, provide a record of climate and environmental change since the end of the last glacial maximum (LGM). Cumbres Bog is unusually deep (basal sediments extend 12 m below the surface) for its altitude (~3050 m a.s.l.) and we extracted 7 m core of continuous sediment below ~5 m of water and peat. The resulting record provides strong evidence of: a period of warming immediately after the LGM (~18–13 cal. kyr BP), a cool interval coinciding with the Younger Dryas (~12.8–11.5 cal. kyr BP), a warm stable period from 10 to 6 cal. kyr BP, and a cooler and highly variable climate interval after 6 cal. kyr BP. More specifically, pollen ratios and fossil diatoms indicate that cold periods generally match with previously identified periods of rapid climate change that occurred at 10.6, 8.7–7.9, 7.0–6.9, 5.4–5.2, 3.3–3.0, 2.3, 2.0 and 1.5 cal. kyr BP. This record also adds resolution to previous regional records and indicates that the periodicity of climate variability changed from 2000–3000 years to 700–1100 years around 6 cal. kyr BP and to <500 years after 3.5 cal. kyr BP. Overall, our record provides important, relatively high-resolution paleoclimatic information for this remote region of the southern Rockies.

Effective river management strategies require an understanding of how fluvial processes vary both spatially and temporally. Here, we examine the natural range of variability in the Conejos River Valley, southern Colorado, through documentation of terrace morphostratigraphic and sedimentological characteristics as well as through investigation of sediment contributions from headwaters, hillslopes and tributary streams. Additionally, soil development and radiocarbon ages, together with local and regional paleoclimate reconstructions, were used to infer the range of processes acting in this system. Since de‐glaciation, the Conejos River has fluctuated between episodes of bedrock strath formation, aggradation and vertical incision. Morphostratigraphic relationships, soil development and radiocarbon ages enable us to propose a chronology for periods of alluvial deposition (around 8·9–7·6 ka, 5·5 ka and from 3·5 to 1·1 ka), separated by intervals of fluvial incision. We infer potential forcing mechanisms by utilizing multiple working hypotheses. Specifically, we discuss the potential for increases in sediment supply during periods of (1) para‐glacial adjustment, (2) climatic cooling, (3) increased frequency of climate change and (4) increased fire frequency or severity. We also consider the effects of changes in stream discharge and extreme storm occurrence. We conclude that combinations of these processes, operating at different times, have contributed to sediment mobilization since de‐glaciation. Stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography. In particular, valley bottom overdeepening at tributary junctions has resulted in incision and strath formation into unlithified glacial deposits (i.e. fill‐cut terraces) rather than bedrock in some reaches. Overall, the Conejos fluvial system has varied significantly both temporally and spatially since de‐glaciation and appears to be sensitive to changes in sediment supply related to Holocene scale climate fluctuations. This natural range of variability must therefore be a key consideration in any future stream management policies. Copyright © 2012 John Wiley & Sons, Ltd.