Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaciären, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.

Investigations of back-barrier, open-coastal plain settings have been used to establish minimum inundation distances of prehistoric tsunamis produced by great subduction zone earthquakes in the central Cascadia margin. Distinctive sand sheets were characterized at four localities within the central Cascadia margin, a shoreline distance of about 250 km. The sand sheets vary in thickness from 0.2 to 25 cm. They thin in the landward direction and consist of well-sorted beach sand that fines upsection. Many of the sand sheets include capping layers of organic-rich detritus, as well as assimilated mud rip-up clasts and soil litter. Marine diatoms and bromine (i.e., marine tracers) were used to confirm marine surge origins for the anomalous sand sheets. Radiocarbon dating of the sand sheets demonstrates correspondence with reported great Cascadia earthquake events at 0.3, ∼1.1, ∼1.3, ∼1.7, and ∼2.5 Ka. One sand sheet mapped at all four localities is dated at 600-950 calibrated radiocarbon years before present. This interpreted paleotsunami event does not correspond to a central Cascadia rupture, so it is tentatively assigned to a far-field source. Minimum overland inundation distances of the near field (Cascadia tsunami) at the four study localities range from 0.3 to 1.3 km, with a mean inundation for all sand sheets of 0.5 km.

To test models of the hydraulics and geometry of englacial conduits, 48 holes (3900m of ice) were drilled into Storglaciaren, Sweden, in search of conduits. About 79% of the holes intersected a hydraulically connected englacial feature. A video camera was used to examine the features and measure local water-flow rates. Because of the extremely clear ice that surrounded most features, their geometry could not be discerned. Of the remaining features, 80% (36) were fracture-like, 16%(6) were of complex geometry, and 4%(2) exhibited a conduit-like geometry. The fracture-like features exhibited steep plunges (∼70˚), narrow openings (∼40mm) and slow water-flow speeds (∼10mms–1). We argue that these fracture-like features are indeed englacial fractures of unknown origin. The depth to fractures intersection varied from near the glacier surface to 96% of local ice depth, with a maximum depth of 131 m. Few hydraulically connected fractures exhibited water motion, indicating some preferential flow pathways exist. We found one ‘traditional’ englacial conduit after an intentional search in a field of moulins. These results suggest that englacial water flow is conveyed through a ubiquitous network of fractures and that conduits are relatively rare.

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The back-barrier wetlands of Cannon Beach, Oregon, record up to six target paleotsunami events in the last three thousand years. The four youngest recorded inundation events are tied to central Cascadia Margin paleotsunami at 1700 AD (event #1), possible farfield paleotsuanmi at 0.8-0.9 ka (event #2), nearfield Cascadia earthquake subsidence at , 1.1 ka (event #3), and nearfield Cascadia earthquake subsidence at , 1.3 ka (event #4). Stratigraphically-correlated deposits from events #1-4 demonstrate strong-flow inundation distances from between 500 m (event #1) and 1,500 m (event #4) from the present beachfront. Based on the spatial distribution and thickness of tsunami sand deposits in the back-barrier wetlands the event #1 paleotsunami did not substantially overtop a stable barrier ridge at 6 m elevation NGVD88 or MLLW at Cannon Beach. The three previous paleotsunami events #2-4 did overtop the barrier, and they deposited sand (1-40 cm thickness) in the back-barrier wetlands. Only paleotsunami event #4 deposited sand across the Ecola Creek upland flood plain, at an inundation distance of at least 1,500 m. Shore-parallel GPR profiles image thin cut-and-fill sequences (1-2 m vertical relief) in the overtopped barrier ridge. A shore-normal GPR profile establishes scour in the wetlands (at least 1.5 m thick cut) at the landward edge of the barrier ridge. The most recent scour is tied to event #2, though earlier paleotsunami overtopping flows might have also caused scour in the back-barrier wetlands. Assuming a 1.0 m water column height and a 1.0 m lower sea level at 1 ka, the three overtopping paleotsunami are estimated to have had minimum runup heights of 8 m relative to mean lower low water.