Amphitheater-headed canyons formed by megaflooding at Malad Gorge,Idaho

Many bedrock canyons on Earth and Mars were eroded by upstream propagating headwalls, and a prominent goal in geomorphology and planetary science is to determine formation processes from canyon morphology. A diagnostic link between process and form remains highly controversial, however, and field investigations that isolate controls on canyon morphology are needed. Here we investigate the origin of Malad Gorge, Idaho, a canyon system cut into basalt with three remarkably distinct heads: two with amphitheater headwalls and the third housing the active Wood River and ending in a 7% grade knickzone. Scoured rims of the headwalls, relict plunge pools, sediment-transport constraints, and cosmogenic (³He) exposure ages indicate formation of the amphitheater-headed canyons by large-scale flooding ∼46 ka, coeval with formation of Box Canyon 18 km to the south as well as the eruption of McKinney Butte Basalt, suggesting widespread canyon formation following lava-flow diversion of the paleo-Wood River. Exposure ages within the knickzone-headed canyon indicate progressive upstream younging of strath terraces and a knickzone propagation rate of 2.5 cm/y over at least the past 33 ka. Results point to a potential diagnostic link between vertical amphitheater headwalls in basalt and rapid erosion during megaflooding due to the onset of block toppling, rather than previous interpretations of seepage erosion, with implications for quantifying the early hydrosphere of Mars.Landscapes adjust to perturbations in tectonics and base level through upstream propagation of steepened river reaches, or knickzones, thereby communicating environmental signals throughout a drainage basin (e.g., ref. 1). Nowhere are knickzones more important and apparent than in landscapes where canyon heads actively cut into plateaus, such as tributaries of the Grand Canyon, United States, and the basaltic plains of Mars (e.g., refs. 2–4). Here the stark topographic contrast between low-relief uplands and deeply incised canyons sharply delineates canyon rims and planform morphology. Canyon heads can have varied shapes from amphitheaters with vertical headwalls to more pointed planform shapes with lower gradients, and a prominent goal in geomorphology and planetary science is to link canyon morphology to formation processes (e.g., refs. 4–8), with implications for understanding the history of water on Mars.Amphitheater-headed canyons on Mars are most likely cut into layered basalt (9, 10), and canyon-formation interpretations have ranged widely from slow seepage erosion to catastrophic megafloods (4–6, 11, 12). Few studies have been conducted on the formation of amphitheater-headed canyons in basalt on Earth, however, and instead, terrestrial canyons in other substrates are often used as Martian analogs. For example, groundwater sapping is a key process in forming amphitheater-headed canyons in unconsolidated sand (e.g., refs. 8, 13, 14), but its importance is controversial in rock (5, 12, 15). Amphitheater-headed canyons are also common to plateaus with strong-over-weak sedimentary rocks (3, 16); however, here the tendency for undercutting is so strong that canyon-head morphology may bear little information about erosional processes, whether driven by groundwater or overland flow (e.g., refs. 3, 5, 17). Canyons in some basaltic landscapes lack strong-over-weak stratigraphy, contain large boulders that require transport, and show potential for headwall retreat by block toppling (18–21), all of which make extension of process–form relationships in sand and sedimentary rocks to basalt and Mars uncertain.To test the hypothesis of a link between canyon formation and canyon morphology in basalt, we need field measurements that can constrain formation processes for canyons with distinct morphologies, but carved into the same rock type. Here we report on the origin of Malad Gorge, a canyon complex eroded into columnar basalt with markedly different shaped canyon heads. Results point to a potential diagnostic link between canyon-head morphology and formative process by megaflood erosion in basalt.Malad Gorge is a tributary to the Snake River Canyon, Idaho, within the Snake River Plain, a broad depression filled by volcanic flows that erupted between ∼15 Ma and ∼2 ka (22, 23). The gorge sits at the northern extent of Hagerman Valley, a particularly wide (∼7 km) part of the Snake River Canyon (Fig. 1). Malad Gorge is eroded into the Gooding Butte Basalt ⁴⁰Ar/³⁹Ar eruption age: 373 ± 12 ka (25)] which is composed of stacked lava beds, each several meters thick with similar well-defined columns bounded by cooling joints and no apparent differences in strength between beds. The Wood (or Malad) River, a major drainage system from the Sawtooth Range to the north, drains through Malad Gorge before joining the Snake River. The Wood River is thought to have been diverted from an ancestral, now pillow lava-filled canyon into Malad Gorge by McKinney Butte basalt flows (24) ⁴⁰Ar/³⁹Ar eruption age: 52 ± 24 ka (25) (Fig. 1).Open in a separate windowFig. 1.Shaded relief map of the study region (50-m contour interval) showing basalt flows (23), their exposure age sample locations, and the path of the ancestral Wood River following Malde (24) (US Geological Survey).Malad Gorge contains three distinct canyon heads herein referred to as Woody’s Cove, Stubby Canyon, and Pointed Canyon (Fig. 2A). Woody’s Cove and Stubby have amphitheater heads with ∼50-m-high vertical headwalls (Fig. 2C), and talus accumulation at headwall bases indicates long-lived inactive fluvial transport (Fig. 3 A and B). Woody’s Cove, the shortest of the three canyons, lacks major spring flows and has minor, intermittent overland flow partially fed by irrigation runoff that spills over the canyon rim. Stubby has no modern-day overland flow entering the canyon, and springs emanate from a pool near its headwall (Fig. 3B). In contrast, Pointed Canyon is distinctly more acute in planform morphology, contains a 7% grade knickzone composed of multiple steps rather than a vertical headwall (Figs. 2C and ​and3C),3C), and extends the farthest upstream.Open in a separate windowFig. 2.Malad Gorge topography (10-m contour interval) and aerial orthophotography (US Geological Survey). (A) Overview map and (B) close-up for Stubby and Pointed canyons showing mapped bedrock scours (white arrows), exposure age sample locations (red circles) with age results, location of the uppermost active knickpoint (black circle), abandoned bedrock channels (blue dashed lines), and grain-size analysis sites (blue squares). The blue star shows the reconstructed location of the headwall of Pointed Canyon at 46 ka (see Discussion and Fig. 5). (C) Longitudinal profile along Stubby and Pointed canyons from their confluence (shown as white lines in B) with local slope, S, averaged over regions demarked by dashed lines (Fig. 4A shows close-up of profile in Stubby Canyon).Open in a separate windowFig. 3.Photographs of (A) headwall of Woody’s Cove (person for scale, circled), (B) ∼50-m-high headwall of Stubby Canyon, (C) downstream-most waterfall at Pointed Canyon knickzone (12-m-high waterfall with overcrossing highway for scale), (D) fluted and polished notch at the rim of Stubby Canyon (notch relief is 10 m), (E) upstream-most waterfall at Pointed Canyon knickzone (within the southern anabranch of Fig. S2), and (F) upstream-most abandoned channel in Fig. 2B and Fig. S2 (channel relief is ∼10 m). White coloring on the headwalls in A and B is likely residue from irrigation runoff.Early work attributed the amphitheater-headed canyons in this region—Malad Gorge, Box Canyon, located 18 km south of Malad Gorge (Fig. 1), and Blue Lakes Canyon located 42 km to the SE—to formation by seepage erosion because of no modern overland flow and the occurrence of some of the largest springs in the United States in this region (7). Because spring flows (e.g., ∼10 m³/s in Box Canyon; US Geological Survey gauge 13095500) are far deficient to move the boulders that line the canyon floors, Stearns (7) reasoned that the boulders must chemically erode in place. This explanation is improbable, however, given the young age of the Quaternary basalt (25), spring water saturated in dissolved solids (19), and no evidence of rapid chemical weathering (e.g., talus blocks are angular and have little to no weathering rinds). Instead of groundwater sapping, Box Canyon was likely carved by a large-scale flood event that occurred ∼45 ka based on ³He cosmogenic exposure age dating of the scoured rim of the canyon headwall (19, 26). In addition, Blue Lakes Canyon was formed during the Bonneville Flood ∼18–22 ka (27, 28)], one of the world’s largest outburst floods that occurred as a result of catastrophic draining of glacial lake Bonneville (21). In both cases, canyon formation was inferred to have occurred through upstream headwall propagation by waterfall erosion.Herein we aim to test whether the amphitheater-headed canyons at Malad Gorge also owe their origin to catastrophic flooding, whether Pointed Canyon has a different origin, and whether canyon morphology is diagnostic of formation process. To this end we present field observations, sediment-size measurements, hydraulic modeling, and cosmogenic exposure ages of water-scoured rock surfaces and basalt-flow surfaces (Methods and Tables S1 and S2).