Influence of multi-decadal land use, irrigation practices and climate on riparian corridors across the Upper Missouri River headwaters basin, Montana.

Melanie K Vanderhoof,Jay R Christensen,Laurie C Alexander

doi:10.5194/hess-23-4269-2019

Abstract

The Upper Missouri River headwaters (UMH) basin (36 400 km2) depends on its river corridors to support irrigated agriculture and world-class trout fisheries. We evaluated trends (1984-2016) in riparian wetness, an indicator of the riparian condition, in peak irrigation months (June, July and August) for 158 km2 of riparian area across the basin using the Landsat normalized difference wetness index (NDWI). We found that 8 of the 19 riparian reaches across the basin showed a significant drying trend over this period, including all three basin outlet reaches along the Jefferson, Madison and Gallatin rivers. The influence of upstream climate was quantified using per reach random forest regressions. Much of the interannual variability in the NDWI was explained by climate, especially by drought indices and annual precipitation, but the significant temporal drying trends persisted in the NDWI-climate model residuals, indicating that trends were not entirely attributable to climate. Over the same period we documented a basin-wide shift from 9 % of agriculture irrigated with center-pivot irrigation to 50 % irrigated with center-pivot irrigation. Riparian reaches with a drying trend had a greater increase in the total area with center-pivot irrigation (within reach and upstream from the reach) relative to riparian reaches without such a trend (p < 0.05). The drying trend, however, did not extend to river discharge. Over the same period, stream gages (n = 7) showed a positive correlation with riparian wetness (p < 0.05) but no trend in summer river discharge, suggesting that riparian areas may be more sensitive to changes in irrigation return flows relative to river discharge. Identifying trends in riparian vegetation is a critical precursor for enhancing the resiliency of river systems and associated riparian corridors.

Highlights

Riparian ecosystems provide critical biological, chemical and hydrological functions (Fritz et al, 2018)
To explore the potential relationship between riparian water storage and river discharge across the Upper Missouri River headwaters (UMH) basin, we identified seven US Geological Survey (USGS) stream gages within the basin, with upstream contributing areas ranging between ∼ 3400 and ∼ 25 000 ha
River length within each riparian reach ranged from 21 km along the Gallatin River to 180 km along the Ruby River and averaged 70 km in length (Table 2; Fig. 1)

Summary

Introduction

Riparian ecosystems provide critical biological, chemical and hydrological functions (Fritz et al, 2018). Defined as semi-terrestrial areas influenced by freshwaters at the interface of rivers and adjacent upland areas (Naiman et al, 2005), riparian ecosystems store water, nutrients and sediments, reducing downstream flood impacts and non-point source pollution (Lowrance et al, 1984; Vivoni et al, 2006) They provide corridors for biotic movement and migration, through arid, urban and agricultural landscapes (Boutin and Belanger, 2003; Lees and Peres, 2008), and maintain fish habitats by lowering stream temperatures and contributing in-stream woody debris (Poole and Berman, 2001; Isaak et al, 2012). Balancing anthropogenic water needs while maintaining or enhancing riparian ecosystem integrity requires an improved understanding of the relationship between water extraction, river discharge and riparian vegetation (Jones et al, 2010; Cunningham et al, 2011)

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Conclusion