Records of bottom pressure at P1 that have been adjusted for changes in the lake’s water level (Fig. 1B, see Methods) show a positive trend over the first 11 days of our experiment, when the lakebed deepened locally by as much as 1.1 m. Throughout this period, five distinct regimes of sediment motion are characterized by local subsidence rates that range between 0 and 0.32 m day−1 (Supp. Fig. 2, Supp. Table 1). Likewise, rapid spikes in bottom pressure are evidence of turbidity currents and underwater landslides, as large concentrations of suspended sediment exert an additional load on our sensor when they transit through P1. We used ADCP and thermistor data taken at P1 and P2 to estimate the contributions made to bottom pressure by variations in the lake’s thermal structure p h and non-hydrostatic effects p nh (see Methods) and subtracted those from our measurements p m . The remaining signal \({p}_{sed}={p}_{m}-({p}_{h}+{p}_{nh})\) can be used to infer the vertically-integrated concentration of sediment above P1 (Fig. 3A, see Methods). Given that suspended matter increases the acoustic reflectivity of water, ADCP measurements of acoustic backscatter further reveal the vertical extent of sediment clouds (Fig. 3B), enabling us to estimate their mean density.

Figure 3 Hydrodynamics of the August 18 landslide: Panel (A) shows fluctuations in bottom pressure attributed to resuspended sediment p sed and hydrodynamic processes \(({p}_{h}+{p}_{nh})^{\prime} \) (left axis). The equivalence of p sed to vertically-integrated sediment concentrations was calculated using \({\rho }_{sed}=1700\mp 300\,{{\rm{kgm}}}^{-3}\) (Eq. 2, see Methods) and is shown on the right axis. ADCP values of acoustic backscatter in panel (B) peak with p sed , confirming enhanced sediment concentrations. Profiles of velocity at P1 (C) and temperature at P2 (D) demonstrate the lake’s interior response to the landslide and subsequent forcing by turbid internal seiches. Full size image

While measurements of bottom pressure and acoustic backscatter demonstrate the occurrence of underwater landslides on June 25 (Fig. 4) and August 18 (Fig. 3), vertical profiles of temperature and flow velocity detail the hydrodynamic processes tied to them. During the August 18 landslide, a turbid front with an approximate mean sediment concentration of 190 ± 50 kg m−3 (Fig. 3A,B) accelerated downslope in Agua Caliente Bay, forcing near-bottom flows and perturbing the lake’s density stratification. As a result, high amplitude IGWs propagated through the lake’s thermocline with periods ranging between 1.1 and 2.4 h (Figs 3D and 5). Peaks that appear at 2.4 h intervals in the time series of p sed reveal that thermocline IGWs conveyed sediment at concentrations near 260 ± 70 kg m−2 even 48 h after their generation (Fig. 3A). Estimates of available potential energy (APE) made using measured temperature (see Methods) indicate that the energy of thermocline oscillations peaked between 4 and 11 × 108 J roughly 10 h after the landslide (Fig. 1C). The spread in our calculations is an effect of assumptions made about the lake’s vertical thermal structure and suggests that as much as 60% of the total observed APE was nonlinear. However, these values necessarily underestimate the true APE, since they don’t account for potential energy stored in sediment clouds, which are coupled to thermocline oscillations and can span the full water column (Figs 3B and 4B).

Figure 4 Hydrodynamics of the June 25 landslide: Panel (A) shows measurements of bottom pressure at P1 that have been adjusted for changes in the water level. Given vertical displacements of our pressure sensor throughout this period, measurements could not be decomposed into components p sed , p h , p nh . Anomalous values of ADCP acoustic backscatter (B) are associated with large disturbances in bottom pressure. These measurements reveal two separate branches of sediment transport, one following the lake’s bottom and another propagating along the thermocline. Profiles of eastward velocity at P1 (C) and temperature at P2 (D) show the lake’s baroclinic response to this landslide. Full size image

Figure 5 Spectrogram of thermistor data. Color shading shows the dominant periods of thermocline IGWs throughout our experiment. At each time, we use 8 days of temperature data to compute power density spectra for each sensor deployed at 14 m depth and plot the spectrum averaged between all instruments. Dashed lines show the theoretical periods \(T=\frac{L}{N}\sqrt{{(\frac{2\pi }{L})}^{2}+{(\frac{n\pi }{h})}^{2}}\) of mode 1 (n = 1) IGWs with wavelengths L = 4150, 3800, 2720 and 2000 m (top to bottom) in a basin with constant depth h = 40 m. Before the landslide of August 18, the temporal variability of measured N2 (Fig. 1D) sufficed to produce a good agreement between theory and observations. After the landslide, however, our fits fail and most IGW energy shifts towards an oscillatory mode best described by L = 4020 m (white line). Full size image

Large (p sed > 0.15 dbar), periodic anomalies in the time series of sediment load (Fig. 3A) show that sediment suspended by the landslide of August 18 persistently returned to P1 in the form of near-bottom, high-density flows. As these basal motions reached Agua Caliente Bay and their pressure signature peaked, return flows intensified above them and ultimately generated a secondary phase of high-frequency thermocline IGWs roughly 7 h after the landslide (Fig. 3C,D). The recirculation of sediment at constant intervals of 6.4 h unequivocally points to the reflection of turbidity currents by the lake’s steep walls. Namely, waves of turbidity traveled back and forth between Agua Caliente Bay and the opposite end of the lake, where topography acted to reverse their propagation. This basin-wide, turbid oscillation remained active for a minimum of four cycles (25.6 h), implying that turbid fronts endured 8 reflections or more. Such temporal persistence requires sediment to propagate in the form of internal seiches along a vertical gradient in the concentration of suspended matter, as turbid solitons cannot endure multiple reflections19.