CHAPTER IX: GRAND CANYON SOILS – A Historical Perspective on Connections with the River

SARA ENDERS

INTRODUCTION While soils develop slowly in the arid environment of the Grand Canyon, they are nonetheless dynamic and revealing. Canyon soils both play an active role in canyon processes and keep passive records of Canyon history, making them valuable both as targets and informants of Glen Canyon Dam Management. Within the Canyon, soils exert direct controls on hydrology and biology. Outside the Canyon, hundreds of miles upstream in the watersheds of tributaries, soil stability or erodibility controls critical inputs to the sediment-starved Colorado River below Glen Canyon Dam. And because of the sensitivity of soil development to environmental conditions, and their “predictable” development over time, soils provide a record of the changing conditions in the Canyon. Such records include clues to historical flow regimes of the Colorado River prior to the construction of Glen Canyon Dam, and times when soils along the river were farmed by ancient peoples.

SOILS OF THE GRAND CANYON, RIM TO RIVER The Grand Canyon is a harsh environment from the perspective of soil formation. Lack of moisture limits the availability of chemical reactants and transport of reaction products, while wind and flooding carry soils away or bury them. The result is that soils of the Grand Canyon tend to be poorly developed, with low amounts of clay and organic matter. Infiltration and drainage are rapid, soil moisture is limiting, and biological productivity is low—which in turn limits the retention of soils on steep canyon slopes.

Soil development (pedogenesis) occurs through the weathering of parent material to secondary products, such as clays and carbonates, and the accumulation of organic matter. In the Grand Canyon, as in all environments, soil development is controlled by five main factors: climate, organisms, topographic relief, parent material, and time (Jenny 1946). As can be seen in a map of Canyon soils (Map 1), soil type in the Grand Canyon varies in bands from the river to the rim, as the river alters topography, climate, and time for formation. Soil development in the Grand Canyon is marked by accumulation of pedogenic carbonate, and the formation of calcic and petrocalcic horizons. At a similar or slower rate is the formation and subsurface concentration of clays, which results upon realization of a threshold of clay accumulation, in the generation of an argillic horizon. These developments are reflected in the naming of soils, which key out taxonomically according to their diagnostic features.

Soils close to the river tend to be least developed. Many soils have insufficient weathering products to form any diagnostic subsurface horizons, this fact governs the taxonomic description and soils are classified as Entisols. If there is incipient subsurface horizon (cambic horizon) formation, then the aridic soil moisture regime is the distinguishing taxonomic feature and soils key out as Aridisols. Farther from the river, where the climate is cooler and moister and pedogenesis has been of longer duration, horizons are sufficiently well developed for soils to key out as Inceptisols, Alfisols, and Mollisols. Where lava flows provide calcium-rich basaltic parent materials, pockets of Vertisols form. The deepest soils are those that developed from alluvium, while soils formed in colluvium and on bedrock are shallower (NRCS, 2003).

RIPARIAN HISTORY IN ALLUVIAL SOILS Riparian soils, formed in alluvial deposits along the Colorado River, record information about the environments of their formation and hold clues to river geomorphology before and after the construction of Glen Canyon Dam.

For the ~5 million years of its history, the Grand Canyon has formed by downcutting of the Colorado River into sediments of the Colorado Plateau (see “Big History” companion volume). This downcutting has been interrupted by periods of aggradation that backfilled the main stem of the river (Lucchitta et al. 2000). For the last ~700 ky, successive intervals of incision and backfilling have stranded a series of surfaces on terraces that increase in age and elevation with distance from the river (Figure 1). Examination of soils on these terraces reveals a sequence of increasing soil development with distance from the river, which aids recognition that these surfaces are different ages. This sequence has been described where it is particularly well-preserved on terrace and hillslope deposits along the Colorado River between Nankoweap and Unkar Creeks and in intervening drainages (Anders et al. 2005; Davis et al. 1995) (see Map 2 for locations).

Holocene-aged terrace surfaces in the Canyon show little to no soil development, lacking a cambic horizon, and key out as Entisols. Soils adjacent to the river tend to be Fluvents, because intermittent burial with alluvium gives them a carbon content that varies irregularly with depth, rather than showing the typical decrease with depth. Beyond the Fluvents are Torripsamments, so named for their dryness, due to a coarse texture and aridic (i.e. torric) soil moisture regime. Beyond these soils, sufficient accumulation of clay and iron and aluminum oxides at depth produces reddish color in the subsurface soil recognizable as a cambic horizon, which takes the soils out of the Entisol order and makes them of the order “Aridisols.” First of the Aridisols in distance from the river are Haplocambids, which show little additional soil development beyond the existence of a cambic horizon. Beyond the Haplocambids, the surface jumps up ~25 meters onto the Pleistocene terrace, and soils show significantly greater development of clay and secondary calcium carbonate. The first soils of this terrace are Petronodic Haplocalcids, in which nodules and concretions of calcium carbonate are common, and calcium carbonate development has reached Stage III. These are followed by soils with sufficient development of clay and carbonate for either or both an argillic horizon and a petrocalcic horizon to form: Duric Haplargids or Argic Petrocalcids (Stage IV carbonate development) (Davis et al. 1995).

Because alluvial and colluvial deposits in this series are well preserved and exposed, their stratigraphic relationships can be resolved (Anders et al. 2005). Extent of pedogenic calcium carbonate can be used to estimate soil ages (e.g. Davis et al. 1995; Lucchitta et al. 2000). Absolute ages can be further constrained with dating by optical luminescence, U-series radiogenic isotope ratios, and cosmogenic-nuclide methods (Anders et al. 2005).

Dates combined with stratigraphic relationships can be used to track the rate of incision of the Colorado River through time, which Pederson et al. 2006 estimate at ~140 m/my for the eastern Grand Canyon by connecting points at which the river was in contact with bedrock over cycles of incision and aggradation. When datable surfaces in the western Grand Canyon are similarly evaluated, they yield lower incision rates. Moreover, gravel deposits in the western Canyon have lower elevation than deposits of the same age in the eastern part of the canyon. This indicates presence of a knickpoint migrating upstream through the Grand Canyon (Lucchitta et al. 2000). These long-term rates provide context in which to view the acceleration of the rate of incision of the Colorado, following construction of Glen Canyon Dam: 11-14m/1000y, or two orders of magnitude greater than the long-term rate (Lucchitta et al. 2000).

The cycles of aggradation-incision recorded in the Pleistocene deposits exhibit Milankovich periodicity and are linkable to glacial-interglacial climate variability. Glacial periods are cooler and wetter, and sediment accumulation along the main stem of the Colorado increases with their onset. Tributary responses, on the other hand, appear to be out-of-phase, with sediment accumulation lagging onset of glacial periods. This has been hypothesized to be due to a two-phase response to increased weathering of Grand Canyon hillslopes (Pederson et al. 2006). Initially, increased weathering results in sediment storage on bare slopes, while transport down-catchment follows later after weathering has exceeded a storage threshold. The lack of delay in the mainstem response reflects responsiveness to headwater hydrology. This suggests that under climate change, we could expect changes in sediment delivery from Grand Canyon hillslopes to show a nonlinear response.

Holocene age terraces at Comanche and Nankoweap creeks provide clues to riparian human settlement starting at least 3100 years ago, on a platform that now sits approximately 10 m above the active flood plain of the Colorado (Davis et al. 2000). These terraces contain buried soils that preserve dark horizons high in organic matter, preserved plant residue, and charcoal—the surface of what used to be agricultural fields. Buried soils at the Comanche site have salinity higher than most field crops can tolerate (101-158dS/m), while neither the parent material nor the overlying alluvial deposits are saline. Salinity of buried soils at the Nankoweap site, by contrast, is only slightly elevated (14-16 dS/m). Pollen of maize and cotton are found at both sites, cultivars that would need irrigation. The salinity of the soils is evidence of irrigation, which causes salts to accumulate over a long period inundation and evaporation. The evidence therefore points to the Comanche site having been farmed with irrigation for a longer period than the Nankoweap site. Agriculture began in the Grand Canyon at a time when the Colorado River was aggrading. When fields became too saline, they would have had to have been abandoned until fresh alluvium was deposited by the river. Cycles of farming and abandonment are indicated in the buried soils. Entrenchment that raised the surface of this terrace to its current Height of 10 m above the river likely would have forced ultimate abandonment of these soils for cultivation, as irrigation would have been made infeasible, ~700 years ago.

POST-DAM CHANGES RIPARIAN AND UPSLOPE STABILITY The construction of Glen Canyon Dam dramatically altered the flow of the Colorado River. Large flood events were effectively eliminated from the hydrograph, and sediment load that would have flowed into the Canyon now falls out behind the dam in Lake Powell. Flows follow regular diurnal patterns, as water is released to time with energy needs (see Burley, this volume). The result for Canyon soils has been increased riparian stability and, potentially, decreased stability of soils on Canyon slopes.

Riparian marshes have increased in abundance since construction of the Dam. Marsh establishment can be shown to correlate with soil texture, itself negatively correlated with daily inundation frequency (Stevens et al. 1995)—more frequent inundation and higher discharge results in coarser soils less favorable for marsh establishment. Marsh density increases downstream in the Canyon, as daily flows become less variable with greater distance from the dam.

On the other hand, alluvial terraces upslope from the riparian corridor appear to be undergoing greater soil erosion than prior to Dam construction. Whether this is due to altered river flow is the subject of debate. Photographs taken in 1973 of archaeological sites at the Holocene terraces have provided a reference against which it is evident that gully incision and erosion has increased in these alluvial terraces over the last several decades (Hereford et al. 1993; Thompson and Potochnik 2000). This could be caused by increased precipitation intensity from the late 1970s through 1990s (Hereford et al. 1993). On the other hand, lowered baselevel of the Colorado River due to dam operations could be a driving factor. Prior to construction of Glen Canyon Dam, floods maintained sediment walls at the mouths of gullies. Changing the flow of the river has altered these sediment plugs, dropping the base level of gullies to the river and causing knickpoint retreat throughout some catchments (Hereford et al. 1993; Thompson and Potochnick, 2000).

Pederson et al. (2006) quantified the extent of erosion on gullied canyon slopes and attempted to find correlates. They noted that median soil-surface shear strength and saturated hydraulic conductivity varied with groundcover: vegetation, soil texture, and the presence of biotic crust or trampling affect likelihood of soil detatchment. The single most important variable, however, was gradient.

SOIL EROSION IN TRIBUTARY CATCHMENTS FEEDS A HUNGRY RIVER Sediment inputs to the Grand Canyon via the Colorado River are valuable resources for terrestrial and aquatic ecosystems, as well as beaches and rapids that facilitate recreation on the River.

Sediment is inputted to Grand Canyon between Glen Canyon Dam and Grand Wash Cliffs via four major tributaries with gaging stations (the Paria and Little Colorado Rivers and the Kanab and Havasu Creeks) and 768 small, ungaged tributaries (Webb et al. 2000). The main tributaries account for the vast majority of new inputs below Glen Canyon Dam: a comparison of the Paria River sediment contribution vs. all other contributions within Glen and Marble Canyon reaches finds that the Paria supplies 80% of all sediment inputs within this reach (Webb et al. 2000).

Given the importance of these tributaries to the sediment budget of the modern Colorado River below Glen Canyon Dam, changes in soil erosion in tributary catchments hundreds of miles from the Colorado River on the plains of Utah and Arizona has the potential to affect this budget. Efforts to recover lands of the Colorado Plateau from effects of overgrazing and unsustainable cropping through the implementation of best management practices have been underway for decades, and there is reason to think that soil erosion in tributary catchments is decreasing.

Commercial cattle ranching began within the Little Colorado catchment in the late 1880s (Abruzzi, 1995), with significant deterioration of the grassland and juniper-piñon woodland communities resulting from overgrazing. Establishment of farming communities exacerbated declines in water quality in the tributaries, where high loads of sediment and total dissolved solids are problematic for aquatic health.

Several reaches of the tributaries are identified as exceeding water quality criteria for these parameters and are therefore the targets of erosion reduction and sediment control (ADEQ 2005, UDEQ, 2005). “Stream sedimentation due to land use activities” is among the four key nonpoint source water quality concerns listed in the watershed based management plan for the Little Colorado River (NEMO, 2005). Protective measures include providing stream bank protection and stabilization, enhancing riparian vegetation along the river and tributaries, and reduced tillage, terrace systems, runoff diversion, filer strips, and conversion of highly erodible cropland to permanent cover away from the riparian corridors. Statewide efforts to control erosion had reduced erosion by 72% in 1997 from 1987 levels (Utah Resource Assessment, 2005). Appreciable reduction in the suspended-sediment discharge since the 1940s has been documented in major tributaries to the Colorado above Lake Powell, the Green River and San Juan River (Gellis et al. 1991).

SUMMARY In sum, knowledge of the sequence by which soil development proceeds in the Grand Canyon allows one to read the history cycles of Colorado River incision in aggradation in assemblages of riparian soils. Current incision rates are two orders of magnitude above long-term rates. Establishment of Glen Canyon Dam increased riparian stability, but has potentially decreased the stability of soils up-slope from the river, threatening the preservation of some archaeological sites. Increased retention of soils within tributary catchments has the potential to reduce sediment inputs to the Colorado River below Glen Canyon Dam, with implications for balancing the river’s sediment budget in the long-term.

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