Okanogan lobe tunnel channels and subglacial floods into Moses Coulee, Channeled Scabland, northwestern United States

The Channeled Scabland, northwestern United States, is a paradigmatic example of outburst flooding that provides insight into megaflood landscapes on Earth and Mars (Gallagher and Bahia, 2021). The scabland landscape, developed in Columbia River Basalts (CRBs), consists of anastomosing bedrock channels containing rock basins, cataracts, and huge gravel bars (Baker, 2009). Current interpretations of the Channeled Scabland attribute formation to megafloods (Q > 10⁶ m³ s^–1) from glacial Lake Missoula (GLM; O'Connor et al., 2020).

Water routing into Moses Coulee, a prominent flood tract of the Channeled Scabland, is enigmatic due to a lack of connectivity to other scabland routes (Bretz, 1923). Moses Coulee heads gradually from the terminal position of the Okanogan lobe of the Cordilleran ice sheet (CIS) on the Waterville Plateau (Fig. 1), suggestive of a connection to Okanogan lobe hydrology (Freeman, 1933). However, the Okanogan lobe may have been an insufficient meltwater source to account for inferred megafloods in Moses Coulee, which thus may have required contributions from a GLM source (Hanson, 1970), routed by overspill or ice-marginal diversion (Waitt, 2021). Geologic evidence needed to distinguish these hypotheses is elusive. Here, we investigated water routing into Moses Coulee by mapping channel networks and glacial landforms.

The Okanogan lobe glaciated the Waterville Plateau during marine isotope stage (MIS) 2, generating glacial landforms (Fig. 2). There is circumstantial evidence for pre–MIS 2 glaciation in the area (Flint, 1935), but no pre–MIS 2 glacial deposits on the Waterville Plateau have been found. The Withrow moraine marks the MIS 2 limit of the Okanogan lobe, north of which are eskers, recessional moraines, and other glacial deposits (Kovanen and Slaymaker, 2004; Hanson, 1970). Varve records and exposure dating bracket the timing of ice cover on the Waterville Plateau to a maximum of 2000–3000 yr (Atwater, 1986) between 17 and 14 ka (Gombiner, 2022). During this time, Okanogan ice impounded glacial Lake Columbia (GLC) and contributed to rerouting of Missoula floods.

Okanogan lobe landforms hint at surge-type behavior, enabled by an integrated hydrologic system (Kovanen and Slaymaker, 2004). Despite inferences that subglacial floods from the CIS contributed to Channeled Scabland erosion (Shaw et al., 1999), evidence for subglacial floods beneath the Okanogan lobe has not been closely examined.

We mapped channels, eskers, and moraines on the former bed of the Okanogan lobe (Figs. 1 and 2) by tracing features on high-resolution digital topography and satellite imagery (U.S. Geological Survey [USGS] 3D Elevation Program 10-m-resolution digital elevation model and Google Earth terrain-enhanced imagery). This work built on prior landform mapping in the area by identifying previously unrecognized features that we compiled into georeferenced landform inventories. We field-checked portions of the channel network to verify channel boundaries and to inspect bedrock surfaces and sediment deposits within and near channels.

Mapping revealed a radial-anastomosing channel network eroded in basalt and sediment, with a minor part in crystalline rock. The network is elongate on the Omak plateau, becoming radial southward toward the Withrow moraine (Fig. 2). Broad trunk channels obliquely join connector channels of smaller dimensions, forming anastomosis. Trunk channels are typically 200–500 m wide and 20–30 m deep, not inclusive of sediment fills, which are on the order of meters to tens of meters in limited well-log reports (WDE, 2023). Boulder lags ornament channel fills (Fig. 3). Channels interweave around stripped bedrock residuals that are increasingly sediment-covered away from channel margins (Fig. 3). Basalt channel morphology is box-like, with stepped channel margins, reflecting the jointed and planar structure of the CRB flows. Crystalline bedrock channels are smaller in all dimensions (Fig. 2). Channels have variable widths and undulatory floors with closed rock basins (tens of meters in depth; Figs. 2 and 3). On the Waterville Plateau, channels climb adverse slopes (Fig. S4 in the Supplemental Material¹) and cross a series of topographic divides (between 650 and 800 + m above sea level [asl]; Fig. 2). The network lacks clear breaks, though this apparent absence may reflect data resolution. At the downstream outlets of the anastomosing network on the Waterville Plateau, smaller channels in bedrock and sediment parallel the Withrow moraine. Some include scabland terrain in basalt (Hanson, 1970), while others contain outwash. These proglacial channels follow the surface slope—unlike the anastomosed network—and converge at the head of Moses Coulee (Fig. 2).

Eskers and recessional moraines overprint portions of the channel network. Eskers (n = 485 mapped features, defined as sinuous ridges of sand and gravel) occur throughout the study area. They typically exhibit single-ridge morphology, with dimensions of hundreds of meters in length, single digits to tens of meters in width, and <20 m in height. Greater numbers of eskers occur in the central-eastern portion of the Waterville Plateau, where some terminate against moraines (Fig. 2). Many eskers occur on channel floors, aligned subparallel to channels. Others are oblique to channels and cross residuals and unchannelized interfluves.

Recessional moraines (n = 404 mapped features, defined as arcuate to sinuous diamict ridges) occur throughout the study area as discontinuous segments (hundreds of meters in length) that become more continuous (kilometers in length) northward. They commonly block channels and drape across topography.

The radial pattern and adverse bed slopes of the tunnel channels are best explained by subglacial flows following hydrostatic pressure gradients under the Okanogan lobe (Lelandais et al., 2016; Supplemental Material D). Additionally, recessional moraines and eskers overprinting channels are a common landform association indicative of tunnel channels (Sharpe et al., 2021).

The coulee morphology and anastomosis of the tunnel channel network are like other areas of the Channeled Scabland, where anastomosis records overwhelming of drainage capacity and progressive channelization (Baker, 2009). The channels of the Omak and Waterville plateaus could be reasonably explained by an analogous process where high-magnitude flows exceeded bankfull capacity in a subglacial environment. Subglacial flows also erode upward into overlying ice, creating additional drainage capacity in ice tunnels, but the existence of the anastomosed tunnel channel network implies that subglacial drainage was not wholly accommodated by ice tunnels. We focused on the tunnel channels eroded downward into the land surface to reconstruct subglacial hydrology, aware of the limitation that additional meltwater may have drained through upward-eroding ice tunnels.

Repeated, low-magnitude flows (5 × 10² to 5 × 10⁴ m³/s; Beaud et al., 2018) may have contributed to erosion of tunnel channels on the Waterville and Omak plateaus, but such flows cannot explain the overall anastomosed network pattern, which requires flows exceeding bankfull capacity. Low-magnitude discharges are typically associated with esker deposition (Lally et al., 2023) and dendritic channel networks that terminate against moraines (Kirkham et al., 2024), while larger floods (10⁴ to >10⁶ m³/s; Kirkham et al., 2019) are associated with anastomosing channels in bedrock (e.g., Lewis et al., 2006), like those on the Waterville and Omak plateaus. Further, low-magnitude flows erode slowly, on time scales of ~7500 yr, to generate channels of comparable dimensions to those mapped here (Beaud et al., 2018); this time scale is longer than the <3000 yr during which the Okanogan lobe covered the Waterville Plateau during MIS 2. Additionally, depositional records of low-magnitude flows (eskers) frequently occur on unchannelized surfaces on the Waterville and Omak plateaus (Fig. 2), implying that esker-forming flows did not erode bedrock channels.

These morphologies and landform associations indicate that subglacial floods eroded the tunnel channel network. The subglacial floods drained at the ice margin into moraine-parallel channels that connected to Moses Coulee (Fig. 2). Scabland in these proglacial channels (Hanson, 1970) suggests conveyance of large outburst floods into Moses Coulee, which plausibly originated as subglacial floods.

The tunnel channel network is likely a palimpsest eroded by flows of varying magnitude, potentially over multiple glaciations. Sedimentary evidence for multiple glaciations in the Okanogan Valley (Lesemann et al., 2013) suggests pre–MIS 2 Okanogan lobes, but pre–MIS 2 deposits of the Okanogan lobe have not been found, so it is unclear when tunnel channel erosion initiated. The absence of breaks in the network suggests simultaneous operation of large sectors, like the Mansfield Channels upstream of Moses Coulee (Hanson, 1970). However, given the hydraulic variability of an active ice lobe, localized operation and/or erosion of smaller subsectors likely occurred.

Connectivity between tunnel channels on the Waterville Plateau and Moses Coulee suggests that subglacial floods drained into Moses Coulee, contributing to coulee erosion and flood deposition. Here, we tested this hypothesis against two prevailing models that route Missoula floods into Moses Coulee in front of the Okanogan lobe: one via back-flooding of Foster Valley and spillover across a 653 m asl divide (Fig. 2, spillover point 1; Waitt, 2021), and the other via NE to SW ice-marginal diversion across the Waterville Plateau (Fig. 2, spillover point 2; O'Connor et al., 2020). An implicit prediction of these models is channel development along flow paths. Analogous ice-marginal diversion has created a distinct channel pattern near Northrup Canyon (ice-marginal spillover channels on Fig. 2). However, no equivalent channel pattern occurs along the proposed diversion path into Moses Coulee, an absence possibly explained by glacial overprinting (Bretz et al., 1956). While faint channelization exists along the proposed diversion paths, the diversion models cannot explain the complete anastomosed tunnel channel network.

Both models that route Missoula floods into Moses Coulee require specific landscape configurations that are incompatible with some geologic evidence and are mutually contradictory (Supplemental Material A). One model requires that upper Grand Coulee was not yet eroded, that the head of Foster Coulee was not yet eroded to its modern elevation, and that the Okanogan lobe selectively blocked the Columbia valley west of the mouth of Foster Creek (Fig. S1; Waitt, 2021). The other model requires that upper Grand Coulee was not yet eroded, that the head of Foster Coulee was eroded to its modern elevation, and that the Okanogan lobe had advanced onto the Waterville Plateau, diverting flow along its margin (Fig. S2; O'Connor et al., 2020). Whether these landscape configurations existed at the time of Moses Coulee floods remains unclear. Some sedimentary records indicate that Grand Coulee breached prior to the last glaciation (Atwater, 1986), contradicting required landscape configurations within both models. Subglacial floods through the tunnel channel network into Moses Coulee are consistent with reconstructed landscape configurations.

The mouth of Moses Coulee preserves sedimentary evidence for four MIS 2 floods (Fig. 1; Waitt, 2021), dated between 17.4 ± 0.8 and 15.5 ± 0.8 ka (Gombiner, 2022). While these floods have been attributed to a GLM source, connectivity between the tunnel channels and Moses Coulee instead implies a subglacial source.

We estimated subglacial discharges into Moses Coulee between 1.3 × 10⁵ m³/s and 5.2 × 10⁶ m³/s. Calculations combined cross-sectional areas of subglacial channels entering the Moses Coulee basin (104,000 m²) with a range of modeled velocities for pressurized subglacial flows (Clarke, 2003), percentage operation of the network, and thicknesses of sediment fill currently in channels (Supplemental Material B; Table S3). These estimates are minima that do not account for discharge through ice tunnels melted into overlying ice. Some estimates exceeded megaflood scale, consistent with the geomorphic inference that tunnel channels conveyed large floods and with reconstructed megaflood discharges in Moses Coulee (O'Connor et al., 2020).

Megaflood discharges through the tunnel channels would require storage and release of water under the Okanogan lobe, consistent with geomorphic evidence upstream. Bedrock tunnel channels along the 300 km length of the Okanogan Valley, including Omak Trench and Soap Lake, indicate subglacial meltwater drainage, and they hydraulically connect the Okanogan lobe to a regional tunnel valley network (Lesemann and Brennand, 2009). Coarse MIS 2 glaciofluvial sediment in the Okanogan Valley has been interpreted as originating in subglacial flows based on thickness, coarseness, and position in overdeepened troughs (Vanderburgh and Roberts, 1996; Eyles et al., 1991), suggesting subglacial floods in the Okanogan Valley. However, identifying records of source reservoirs is challenging due to a lack of diagnostic subglacial criteria for glacial lake deposits (Livingstone et al., 2012) and the low preservation potential of such subglacial records. Considering these limitations, we explored mechanisms for water storage and release in the Okanogan Valley.

In the Okanogan Valley, ice advance over lakes could have formed “catch lakes” (Rudoy, 1998) during ice expansion. Once covered by ice, high local geothermal heat flux could have enhanced meltwater production (Lesemann and Brennand, 2009), and the overdeepened valley would have favored development of a hydraulic potential low and formation of a subglacial reservoir (Livingstone et al., 2013). Initiation of subglacial reservoir drainage would have required a trigger, such as a change in thermal conditions (Skidmore and Sharp, 1999) or an abrupt input of water. Such water inputs into the Okanogan Valley could have resulted from drainage of supraglacial lakes, of ice-marginal lakes, or of subglacial lakes farther up the ice sheet. Hydraulic pressure from these lakes may have initiated drainage cascades into the Okanogan Valley, which operated as part of a water storage and transfer network, like bedrock tunnel channels in formerly glaciated areas of West Antarctica (Kirkham et al., 2019).

Moses Coulee has been enigmatic due to its lack of clear connectivity to the main Channeled Scabland flood routes. We resolved this enigma by identifying an anastomosing network of subglacial channels on Waterville and Omak plateaus, Washington State, northwestern United States, which records subglacial meltwater floods beneath the Okanogan lobe and into Moses Coulee. These subglacial channels are analogous to tunnel channels in other glaciated areas. Water sources to supply these channels could have included drainage of supraglacial lakes and linked ice-marginal and subglacial reservoirs. During MIS 2 glaciation, meltwater was routed into Moses Coulee via subglacial anastomosed channels. Prevailing Missoula flood spillover hypotheses remain theoretically viable but depend upon unique configurations of Grand Coulee and the Okanogan lobe, and they are difficult to reconcile with the observed channel pattern. These conclusions emphasize the importance of Cordilleran ice sheet (sub-)glacial hydrology to the formation of the Channeled Scabland (Shaw et al., 1999).

We thank Brian Atwater, Nick Zentner, Skye Cooley, and John Stone for helpful discussions and encouragement, and we thank Vic Baker and two anonymous reviewers for constructive feedback. We dedicate this article to the memory of John Shaw. J. Gombiner thanks the Geological Society of America Graduate Student Research fund for analytical support.