The Bonito Paleochannel in Chaco Canyon, New Mexico: Recent Research and Implications for Causality and Effects

AbstractDuring the late eleventh century AD, a episode of erosion in Chaco Canyon, New Mexico, threatened to destroy portions of the Pueblo Bonito great house and possibly other large buildings. Known as the Bonito Paleochannel, this large arroyo meant the loss of significant amounts of surface water from valley-wide flooding originating in upstream and tributary sources. We believe that erosion was probably the result of an increasing size and frequency of floods. There is no compelling evidence that agriculture was affected by the paleochannel but we hypothesize that great house construction became extremely difficult. Consequently, the impact of the Bonito Paleochannel cycle may have been felt most in the political economy.A fines del siglo XI d. C., un episodio de erosión en el Cañón del Chaco, Nuevo México, amenazó con destruir partes de la gran casa de Pueblo Bonito y posiblemente otros edificios grandes. Conocido como el Paleocanal Bonito, este gran arroyo significó la pérdida de cantidades significativas de agua superficial debido a las inundaciones en todo el valle que se originaron en las fuentes río arriba y tributarias. Creemos que la erosión fue probablemente el resultado de un aumento en el tamaño y la frecuencia de las inundaciones. No hay evidencia convincente de que la agricultura se haya visto afectada por el paleocanal, pero planteamos la hipótesis de que la construcción de grandes casas se volvió extremadamente difícil. En consecuencia, el impacto del ciclo Bonito Paleochannel puede haberse sentido más en la economía política.KEYWORDS: Chaco CanyonBonito phasegreat housesBonito Palecochannelarroyo cyclepluvial periodmegadroughtpolitical economy AcknowledgementsField investigations and laboratory analyses were conducted under permits from Chaco Culture National Historical Park, United States National Park Service with funding from the National Science Foundation (BCS 1523224). David W. Love has provided critical geological insight throughout our research program and we have included some of his own original research in our Supplemental Material, but he is in no way responsible for any errors or misunderstandings in this article. Our deepest appreciation to Dabney Ford, Roger Moore and Wendy Bustard, as well as the many students who participated in the fieldwork. The authors are especially grateful to Gary Huckleberry and two anonymous reviewers for exceptionally close readings of the original manuscript and providing important insights and corrections.Disclosure StatementNo potential conflict of interest was reported by the author(s).University of New Mexico People’s Land and Territory AcknowledgementFounded in 1889, the University of New Mexico sits on the traditional lands of the Pueblo of Sandia. Since time immemorial, the original peoples of New Mexico – Pueblo, Navajo and Apache – have deep connections to the land and have made significant contributions to the broader community statewide. We honor the land throughout the generations and acknowledge our committed relationship to Indigenous peoples. We gratefully recognize our history.Notes1 Recently, Lentz, et al. (Citation2021:1) argued that during the Puebloan occupation at Chaco “the local woodlands, especially the juniper component, had been decimated by centuries of continuous extraction of a slow-growing resource” based mainly on seven pollen samples obtained from two test pits in alluvium at the extreme western end of the canyon in which they identify a decline in juniper pollen percentages from 12-20% prior to 600 BC to 4-8% at AD 600 and 2% at AD 1100. Hall (Citation1977, Citation2010a) previously identified a significant decline in juniper after ca. 500 BC, but not a subsequent temporal trend, as individual samples from different locations during the Puebloan period (ca. AD 400 to AD 1250) range between 0 and 8% (only three exceeded 4%), which he interpreted as reflecting a persistent low-density juniper presence similar to current conditions, as modern surface pollen samples exhibit the same range of low percentages (see Supplemental Document 1, pp. 6-12). Hall and other palynologists and geologists (Cully Citation1985, Citation1988; Cummings Citation2001; Fredlund 1986; Love Citation1980:396; Smith Citation2020) working at Chaco have been careful to note a variety of taphonomic issues (e.g., facies, aeolian deposition, reworked sediments, isolated nearby trees) that may account for variation in small pollen percentages within sample columns and other contexts. Given the unknown ecological and spatial structure of local pinyon-juniper woodlands prior to and during the Bonito Phase (Love Citation1980:398; Wills, Drake and Dorshow Citation2014; see also Vivian et al. Citation2006), any inferences about the role of people in removing junipers and instigating “destabilizing impacts” (Lentz et al. Citation2021:1) are speculative. Recent studies in the US Southwest indicate that juniper - often considered tolerant of drought conditions - is susceptible to dieback during droughts (Hartsell et al. Citation2020; Kannenberg et al. Citation2021; Pangle et al. Citation2015; Romme et al. Citation2009). Therefore, disentangling cultural and natural impacts on past vegetation at Chaco remains an important but unresolved issue. We encourage readers who are interested in the research about Chaco vegetation history to consult Betancourt and Van Devender (Citation1983), Hall (Citation1977, Citation1988, Citation1990a; Citation1997, Citation2010a, Citation2010b), Cully (Citation1985, Citation1988), Gillespie (Citation1985), Love (Citation1980); Mathien (Citation2005), Toll (Citation1985), Smith (Citation2020), Vivian et al. (Citation2006) and Wills et al. (Citation2016).