A Tale of Two Rivers: The Role of Different Drought-Like Conditions in Promoting Vegetation Encroachment on the Lower Dolores River
Cynthia Dott1* and Alan Kasprak2
1Fort Lewis College Dept of Biology, Durango, CO; dott_c@fortlewis.edu
2Fort Lewis College Dept of Geosciences, Durango, CO; akasprak@fortlewis.edu
The effects of river regulation on the flow patterns of rivers in western North America often mimic the impacts of naturally-occurring episodic drought. On the Dolores River in southwestern Colorado, two distinct periods of flow modification have occurred, with very different drought-like consequences for downstream habitat. The first period of major irrigation diversions (1889-1984), led to extreme low flows during the summer months with little change to spring peak flows. The second period after dam construction (1984-present), caused a decline in peak flows but an increase in summer minimum flows. We studied the role of these different flow regimes in promoting the recruitment of non-native tamarisk (Tamarix spp.) and native coyote willow (Salix exigua). We used dendrochronology to determine tamarisk establishment dates and tested the connection between establishment years and a suite of hydroclimatic variables. Tamarisk recruitment occurred between 1952-2002, with 92% of trees establishing during the first, flow diversion period. The strongest hydroclimate predictor was low minimum flow for a given year, especially when coupled with higher peak flows in the year of or before establishment. Almost no tamarisk recruitment has occurred in the second, dam-controlled period, where lowered peak flows but elevated minimum flows prevail. Instead, the combination of lowered peaks but increased summer baseflows post-dam has driven more rapid channel narrowing and vegetation encroachment by coyote willow. In both cases, aspects of flow modification that mimic drought led to dramatic changes in vegetation composition, and in the “second river” period this has had major impacts on in-channel habitat conditions. Given current and future drought conditions on western rivers, water managers and researchers need to be vigilant for potential new invasions and for threshold-crossing vegetation change that could impact wildlife diversity in both riparian and aquatic habitats.
Read the article here: https://doi.org/10.1002/eco.2429
Dott, C., Bombaci, S., Kasprak, A., & Korb, J. (2022). A tale of two rivers: Dam-induced hydrologic drought on the lower Dolores River and its impact on tamarisk establishment. Ecohydrology, 15( 6), e2429.