Physical processes involved in recent activity within the moon

The three phenomena which indicate activity within the moon at present, moonquakes, emissions of Rn and Ar and the enigmatic lunar transient phenomena (LTP) possess some spatial and temporal correlation. The moonquakes and LTP occur at apogee and perigee and the gas emissions are episodic. All three have some connection with the maria regions of the moon. It is suggested that gradual settling of the mascons provides the energy released in moonquakes — the tides being a trigger. The fault systems around the circular maria allow the escape of the radioactive daughter products and probably volatiles when the cracks are opened at apogee and perigee.     

Creep response of the lunar crust in mare regions from an analysis of crater deformation

The settling trends of 318 lunar mare craters are compared with predictions of numerical finite-element models in order to determine the creep response of the upper lunar mare crust. No settling is evident in craters smaller than 5 km in diameter. Settling rates of larger craters increase as function of crater size in a manner suggesting a non-linear lunar creep response corresponding to the power law $\text{ε}\text{?}\text{= 8.3 · 10}{}^{\mathrm{?34}}\text{σ}{}^2$ where έ is the strain rate and σ is the differential stress. However, the observed nonlinearity is probably an apparent nonlinearity resulting from the temperature induced viscosity decrease with depth due to a lunar crustal temperature gradient of 3° C/km and a creep activation energy of 20 kcal/mole. It is concluded that creep in the lunar medium is essentially Newtonian, and that the effective viscosity of the upper lunar mare crust is (1.6 ± 0.3) · 10²⁵ poise.     

Primeval displacements of the lunar pole

The hypothesis that the magnetic field which magnetized the lunar crust was generated by the dynamo process in a small fluid iron core can now be tested. Because the Coriolis force was a dominant term in the equations of motion in this core, the mean lunar field was aligned along the ancient axis of rotation. From Hood's modelling of the magnetic anomalies in the lunar crust, mapped by the Apollo 15 and 16 subsatellites, the palaeo-directions of this field have been determined. From them, palaeopole positions have been determined and are found to be grouped with respect to age. The palaeoequators corresponding to ages 4 Ga and 3.85 Ga show close relationships with the circular maria or mascons on the near side and multi-ring basins of corresponding age on the far side. The polar displacements indicated from lunar palaeomagnetism can be explained by the changes in the moment of inertia tensor consequent on the excavation and later flooding of these circular mare. Small moons in the primeval Earth-Moon system are inferred to be the impacting bodies.     

Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

Apollo 15 and 16 subsatellite measurements of lunar surface magnetic fields by the electron reflection method are summarized. Patches of strong surface fields ranging from less than $\text{1}\text{4}$° to tens of degrees in size are found distributed over the lunar surface, but in general no obvious correlation is observed between field anomalies and surface geology. In lunar mare regions a positive statistical correlation is found between the surface field strength and the geologic age of the surface as determined from crater erosion studies. However, there is a lack of correlation of surface field with impact craters in the mare, implying that mare do not have a strong large-scale uniform magnetization as might be expected from an ancient lunar dynamo. This lack of correlation also indicates that mare impact processes do not generate strong magnetization coherent over ～ 10 km scale size. In the lunar highlands fields of >100 nT are found in a region of order 10 km wide and >300 km long centered on and paralleling the long linear rille, Rima Sirsalis. These fields imply that the rille has a strong magnetization (>5 × 10^?6 gauss cm³ gm^?1 associated with it, either in the form of intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock. However, a survey of seven lunar farside magnetic anomalies observed by the Apollo 16 subsatellite suggests a correlation with inner ejecta material from large impact basins. The implications of these results for the origin of lunar magnetism are discussed.     

Radon emanation as an indicator of current activity of the moon

An interpretation of previously reported measurements of the Apollo 15/16 alpha-particle spectrometer on the distribution of ²²²Rn and ²¹⁰Po across the lunar surface suggests that continuation of these measurements is a method of monitoring current activity on the moon. Since the two isotopes are relatively short-lived with effective half-lives of 3 days and 21 years, respectively, the activity detected has had to have been released during this current epoch. Changes in the rate of lunar emanation can be measured on three different time scales: (1) of a few days or less by detecting ²²²Rn at discrete sites such as the crater Aristarchus; (2) of a month by measuring ²²²Rn activity at the sunrise terminator; (3) of a few years by measuring ²¹⁰Po activity at various locations. These observations could be carried out very effectively from a lunar polar orbiting satellite.     

Density and composition of interplanetary dust particles

Residual meteoritic material has been detected on the surface of crater interiors on lunar samples 60315,29 and 65315,68. Iron-nickel micrometeoroid residues are present in the form of mettallic spherules embedded in or attached to the crater glass-linings; stony-iron meteorite residual material is homogeneously mixed with the glass-linings.Crater simulation experiments show the dependence of crater diameter to depth ratio on projectile density. On the other hand, the projectile velocities exceeding 4 km/s have no measurable influence on the D/T ratios of microcraters. As a result, diameter-to-depth measurements on lunar microcraters yield three groups of micrometeorites in the size range between 1 μm and 1 mm: iron-nickel, stony-iron and low-density particles. The measured D/T values correspond directly to the kind of determined projectile residues: craters showing residues of iron-nickel meteorites have ratios of D/T = 1.3–1.4 and craters with stony-iron residues have ratios ofD/T = 1.9–2.1.Craters with diameters ?30 μm seem to have been formed predominantly by iron-nickel micrometeorites, whereas craters with diameters ?80 μm predominantly by stony meteorites.     

Lunar magnetic anomalies detected by the Apollo substatellite magnetometers

Properties of lunar crustal magnetization thus far deduced from Apollo subsatellite magnetometer data are reviewed using two of the most accurate presently available magnetic anomaly maps — one covering a portion of the lunar near side and the other a part of the far side.

The largest single anomaly found within the region of coverage on the near-side map correlates exactly with a conspicuous, light-colored marking in western Oceanus Procellarum called Reiner Gamma. This feature is interpreted as an unusual deposit of ejecta from secondary craters of the large nearby primary impact crater Cavalerius. An age for Cavalerius (and, by implication, for Reiner Gamma) of 3.2 ± 0.2 × 10⁹ y is estimated. The main (30 × 60 km) Reiner Gamma deposit is nearly uniformly magnetized in a single direction, with a minimum mean magnetization intensity of 7 × 10⁻² G cm³/g (assuming a density of 3 g/cm³), or about 700 times the stable magnetization component of the most magnetic returned samples. Additional medium-amplitude anomalies exist over the Fra Mauro Formation (Imbrium basin ejecta emplaced 3.9 × 10⁹ y ago) where it has not been flooded by mare basalt flows, but are nearly absent over the maria and over the craters Copernicus, Kepler, and Reiner and their encircling ejecta mantles.

The mean altitude of the far-side anomaly gap is much higher than that of the near-side map and the surface geology is more complex, so individual anomaly sources have not yet been identified. However, it is clear that a concentration of especially strong sources exists in the vicinity of the craters Van de Graaff and Aitken. Numerical modeling of the associated fields reveals that the source locations do not correspond with the larger primary impact craters of the region and, by analogy with Reiner Gamma, may be less conspicuous secondary crater ejecta deposits. The reason for a special concentration of strong sources in the Van de Graaff-Aitken region is unknown, but may be indirectly related to the existence of strongly modified crustal terrain which also occurs in the same region. The inferred directions of magnetization for the several sources of the largest anomalies are highly inclined with respect to one another, but are generally depleted in the north-south direction. The north-south depletion of magnetization intensity appears to continue across the far-side within the region of coverage.

The mechanism of magnetization and the origin of the magnetizing field remain unresolved, but the uniformity with which the Reiner Gamma deposit is apparently magnetized, and the north-south depletion of magnetization intensity across a substantial portion of the far side, seem to require the existence of an ambient field, perhaps of global or larger extent. The very different inferred directions of magnetization possessed by nearly adjacent sources of the Van de Graaff-Aitken anomalies, and the depletion in their north-south component of magnetization, do not favor an internally generated dipolar field oriented parallel to the present spin axis. A variably oriented interplanetary magnetizing field that was intrinsically strong or locally amplified by unknown surface processes is least inconsistent with the data.     

Radial thickness variation in impact crater ejecta: implications for lunar basin deposits

A review of cratering data and available semi-empirical calculations suggests that the variation of ejecta thickness,t, with increasing range from lunar craters may be approximately modelled by the expression: t=0.14R^0.74(r/R^?3.0 wherer is range from the center of the crater andR, the crater radius, all in meters. This equation has been used to estimate the thickness of ejecta deposits at each of the Apollo sites contributed from the large multi-ringed frontside lunar basins. Predicted average thickness of Imbrium ejecta at Apollo 15 is 812 m; at Apollo 14, 130 m; at Apollo 17, 102 m; and at Apollo 16, 50 m. Since the sequence of formation of these basins is known, the stratigraphic column resulting from superimposed ejecta blankets can be calculated. Results suggest that pre-Nubium crustal material at upland Apollo sites lies at depths greater than 280 (Apollo 14) to 1940 m (Apollo 17). Predicted stratigraphic sections for the Apollo sites are tabulated.     

Mondkraterformationen auf der Erde

Zusammenfassung Es wird gezeigt, dass wir einige irdische Vulkankrater kennen, deren morphologischer Aufbau nahezu identisch ist mit den typischen Formen der Mondkrater. Auf die Wichtigkeit dieses Befundes für das Problem der Entstehung der Mondkrater wird hingewiesen. Es ist zu vermuten, dass sich auf der Erde noch weitere Beispiele dieser Art werden finden lassen.

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Summary It is shown, that some terrestrial volcanic craters are known, the morphological structure of which is practically identical with the typical shapes of lunar craters. The importance of this relation for the problem of the origin of lunar craters is pointed out. It may be expected, that some more examples of this kind might be found.

     

Argon-lead isotopic correlation in samples from lunar maria: Records from the ancient lunar regolith

²⁰⁷Pb/²⁰⁶Pb of “low temperature sited” (LTS) lead as reported by Silver (1975) increases with⁴⁰Ar/³⁶Ar of trapped argon in thirteen samples from lunar maria. This strongly supports an earlier conclusion by (1972) that large (⁴⁰Ar/³⁶Ar)_T ratios represent ancient regolith records, and provides a rough (⁴⁰Ar/³⁶Ar)_T timescale.The erasure of (⁴⁰Ar/³⁶Ar)_T records in surface soils by the excavation of deep-seated, “fresh” bedrock and by erosion of particle surfaces via ion sputtering must have been counteracted by conserving processes in the regolith. Two such processes are relatively well understood: agglutinate formation and the excavation and comminution of soil breccias which have preserved an ancient (⁴⁰Ar/³⁶Ar)_T record. The frequency distribution of (⁴⁰Ar/³⁶Ar)_T in 82 “soils” from all Apollo missions suggests a third process, which requires that sizeable “pockets” of ancient regolith materials including soils have survived deep turnover for billions of years.Large-scale mobility of LTS lead throughout all of the regolith does not appear to occur.Inert gas ions with sufficient energy for trapping may have reached the lunar surface more than 3 b.y. ago.The Apollo 11 microbreccias appear to have been formed more than 3 b.y. ago from regoliththen extant on the surface.     

Impact flows and crater scaling on the moon

The axisymmetric distribution of stress, internal energy and particle velocity resulting from the impact of an iron meteoroid with a gabbroic anorthosite lunar crust has been calculated for the regime in which shock-induced melting and vaporization takes place. Comparison of impact flow fields, with phase changes in silicates taken into account, with earlier results demonstrate that in the phase change case when the 15-km/s projectile has penetrated some two projectile radii into the moon, the peak stress in the flow is ～0.66 Mbar at a depth of 66 km, and the stress has decayed to ～66 kbar at a depth of 47 km. Rapid attenuation occurs because of the high rarefaction velocity of the high-pressure phases associated with a 35% (zero-pressure) density increase. This feature of the phase-change flow tends to strongly concentrate the maximum shock pressures along the meteoroid trajectory (axis) and makes the conical zone along which high internal energy deposition occurs, both shallow and narrow. Examination of the gravitational energies required to excavate larger craters on the moon indicates the importance of gravity forces acting during the excavation of craters having radii in the range greater than ～2 – ～140 km. It is observed that the “hydrodynamic” energy vs. crater radius relation approaches those for various “gravitational” energy vs. radius relations at the radii values corresponding to the larger mare basins. Cratering energy values in the range of (1.0 – 9.4) · 10³² erg are inferred on this basis for the Imbrium crater. Using these values and the criteria that all rocks exposed to ～100 kbar or greater shock pressures are included in the ejecta (some of which falls back) implies that the maximum depth of sampling expected to be represented within the Apollo collection lies in the range 148–328 km.     

Ejecta from large craters on the Moon: Comments on the geometric model of McGetchin et al.

Amendments to a quantitative scheme developed by T.R. McGetchin et al. (1973) for predicting the distribution of ejecta from lunar basins yield substantially thicker estimates of ejecta, deposited at the basin rim-crest and at varying ranges byond, than does the original model. Estimates of the total volume of material ejected from a basin, illustrated by Imbrium, also are much greater. Because many uncertainties affect any geometric model developed primarily from terrestrial analogs of lunar craters, predictions of ejecta thickness and volume on the Moon may range within at least an order of magnitude. These problems are exemplified by the variability ofT, thickness of ejecta at the rim-crest of terrestrial experimental craters. The proportion ofT to crater rim-height depends critically upon scaled depth-of-burst and whether the explosive is nuclear or chemical.     

Depth profile of53Mn in the lunar surface

Measurements of cosmic-ray produced⁵³Mn are reported for a series of lunar surface samples down to a depth of 416 g/cm². These results clearly illustrate the decrease in activity with depth as the incident galactic cosmic rays are absorbed. Below 60 g/cm² the production rate decreases exponentially with a mean length, λ, of about 220 g/cm². These results indicate that, at the Apollo 15 site, the lunar regolith has been unmixed, on a meter scale, for the last 5 my. The neutron activation technique for⁵³Mn, which allowed samples smaller than 200 mg to be used for these measurements, is described.     

Studies on the thermoluminescence depth profile produced by 600-MeV protons in artificial lunar soil

The thermoluminescence (TL) of various plagioclase feldspars embedded in a thick target of 150 kg of artificial lunar soil was measured after a 600-MeV proton irradiation. No correlation was observed between the parameters of the characteristic feldspar glow peak and the anorthite contents. The relative TL sensitivities of the individual plagioclase variants were measured and found to be practically the same for⁶⁰Co-γ- and 600-MeV proton-irradiated samples.The TL intensity distribution within the target arrangement, converted to a 2π isotropic p-influx, resulted in an approximate TL depth profile of a thermally undisturbed lunar soil bomarded by galactic cosmic protons. The undisturbed TL intensity at a depth of 28 g/cm² (? 17 cm) decreased to 39% at a depth of 106 g/cm² (? 60 cm). For the evaluation of the temperature gradients by TL in lunar samples the experimental data at the sites of Taurus-Littrow and of Hadley-Rille yielded minimum depth intervals for sampling of ～ 20 cm and ～ 40 cm respectively, presuming an error of ± 15% in the TL determination. Certain aspects are seen by using the relation TL intensity/²²Na-activity ratio versus depth (thus representing the total ionization profile) to establish²²Na depth profiles.     

Crustal structure, evolution, and volcanic unrest of the Alban Hills, Central Italy

 The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15–25 km southeast of Rome, last erupted 19 ka and has produced approximately 290 km³ of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably at least 40×10⁶ m³ and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity. Received: 15 April 1997 / Accepted: 9 August 1997     

Lunar seismicity and tectonics

Seismic signals from 300–700 deep moonquakes and about four shallow moonquakes are detected by the long-period seismometers of two or more of the Apollo seismic stations annually. Deep-moonquake activity detected by the Apollo seismic network displays tidal periodicities of 0.5 and 1 month, 206 d and 6 a. Repetitive moonquakes from 60 hypocenters produce seismograms characteristic of each. At each hypocenter, moonquakes occur only within an active period of a few days during a characteristic phase of the monthly lunar tidal cycle. An episode of activity may contain up to four quakes from one hypocenter. Nearly equal numbers of hypocenters are active at opposite phases of the monthly cycle, accounting for the 0.5-month periodicity. The 0.5- and 1-month activity peaks occur near times of extreme latitudinal and longitudinal librations and earth-moon separation (EMS). The 206-d and 6-a periodicities in moonquake occurrence and energy release characteristics are associated with the phase variations between the librations and EMS. Because of the exact relationship between tidal phases and the occurrence of deep moonquakes from a particular hypocenter, it is possible to predict not only the occurrence times from month to month, often to within several hours, but also the magnitudes of the moonquakes from that hypocenter. The predicted occurrence of large A₁ moonquakes in 1975, following a 3-a hiatus, confirms the correlation between A₁-moonquake activity and the 6-a lunar tidal cycle and implies a similar resurgence for all of the deep moonquakes. Because no matching shallow moonquake signals have been identified to date, tidal periodicities cannot be identified for the individual sources. However, shallow moonquakes generally occur near the times of extreme librations and EMS and often near the same tidal phase as the closest deep moonquake epicenters. With several possible exceptations, the deep-moonquake foci located to date occur in three narrow belts on the nearside of the moon. The belts are 100–300 km wide, 1,000–2,500 km long and 800–1,000 km deep and define a global fracture system that intersects in central Oceanus Procellarum. A fourth active, although poorly defined, zone is indicated. The locations of 17 shallow-moonquake foci, although not as accurate as the deep foci, show fair agreement with the deep-moonquake belts. Focal depths calculated for the shallow moonquakes range from 0–200 km. Deep-moonquake magnitudes range from 0.5 to 1.3 on the Richter scale with a total energy release estimated to be about 10¹¹ erg annually. The largest shallow moonquakes have magnitudes of 4–5 and release about 10¹⁵–10¹⁸ erg each. Tidal deformation of a rigid lunar lithosphere overlying a reduced-rigidity asthenosphere leads to stress and strain concentrations near the base of the lithosphere at the level of the deep moonquakes. Although tidal strain energy can account for the deep moonquakes in this model, it cannot account for the shallow moonquakes. The tidal stresses within the lunar lithosphere range from about 0.1 to 1 bar and are insufficient to generate moonquakes in unfractured rock, suggesting that lunar tides act as a triggering mechanism. The largest deep moonquakes of each belt usually occur near the same characteristic tidal phases corresponding to near minimum or maximum tidal stress, increasing tidal stress, and alignments of tidal shear stresses that correspond to thrust faulting along planes parallel to the moonquake belts and dipping 30–40°. With few exceptions, the shallow moonquakes occur at times of near minimum tidal stress conditions and increasing tidal stress that also suggest thrust faulting. The secular accumulation of strain energy required for the shallow moonquakes and implied by the uniform polarities of the deep moonquake signals probably results from weak convection. A convective mechanism would explain the close association between moonquake locations and the distribution of filled mare basins and thin lunar crust, the earth-side topographic bulge, and the ancient lunar magnetic field. The low level of lunar seismic activity and the occurrence of thrust faulting both at shallow and great depths implies that the moon is presently cooling and contracting at a slow rate.     

Oxygen isotope measurements of phosphate from fish teeth and bones

In situ measurements of lunar surface brightness temperatures made as a part of the Apollo Lunar Surface Experiments Package at the Apollo 15 Hadley Rille landing site are reported. Data derived from 5 thermocouples of the Heat Flow Experiment, which are lying on or just above the surface, are used to examine the thermal properties of the upper 15 cm of the lunar regolith using eclipse and nighttime cool-down temperatures. Application of finite-difference techniques in modeling the lunar soil shows the thermocouple data are best fit by a model consisting of a low-density and low-thermal conductivity surface layer approximately 2 cm thick overlying a region increasing in conductivity and density with depth. Conductivities on the order of 1 × 10^?5 W/cm-°K are postulated for the upper layer, with conductivity increasing to the order of 1 × 10^?4 W/cm-°K at depths exceeding 20 cm. An increase in mean temperature with depth indicates that the ratio of radiative to conductive transfer at 350°K is 2.7 for at least the upper few centimeters of lunar soil; this value is nearly twice that measured for returned lunar fines. The thermal properties model deduced from Apollo 15 surface temperatures is consistent with earth-based microwave observations if electrical properties measured on returned lunar fines are assumed.     

Velocity-density systematics,mean atomic weight,and the interior of the moon

Mean atomic weight profiles for the lunar mantle have been calculated from velocity-density systematic relations using lunar density and seismic velocity models. Despite large variability among the models, the calculation including Poisson's ratio yields a range of mean atomic weight values between 22 and 23 g mol^?1 below 150 km. A similar calculation for the Earth's mantle produces a mean atomic weight of 21.1 ±0.4 g mol^?1. This suggests that the Moon cannot be derived directly from the Earth's mantle, or that it has had a differentiation history different from the Earth's. The lunar $\text{m}\text{'}\text{s}$ require an Fe mole fraction between 0.25 and 0.33 for a pure olivine mantle, or between 0.33 and 0.45 for pure pyroxene.The present profiles are 0.5–3.0 g mol^?1 higher than those calculated from lunar compositional models based on lunar rock compositions and petrology and assumed lunar histories, indicating inadequacies in either the seismic or compositional models, or in both. The mean atomic weight approach provides a method of comparing the consistency of seismic and compositional models of planetary interiors.     

SmNd constraints on early lunar differentiation and the evolution of KREEP

The Sm-Nd systematics of lunar KREEP basalt 15386 reflects two chronologically distinct events in the development of the incompatible element-rich materials of the moon. The measured Sm-Nd mineral isochron of 15386 indicates an age of 3.85 ± 0.08 AE which is consistent with the reported Rb-Sr and³⁹Ar-⁴⁰Ar ages of many other KREEP-rich samples. This age is interpreted as the time at which 15386 crystallized from a liquid on or near the lunar surface. The frequent occurrence of this age for KREEP-dominated samples, as well as the restricted location of KREEP near major lunar near-side impact basins, suggests that the eruption of these incompatible element-rich liquids was related to deep impact events during the postulated final bombardment phase of the surface of the moon. However, the lower than chrondritic initial¹⁴³Nd/¹⁴⁴Nd of 15386 and the essentially identical Sm-Nd evolution of other KREEP-rich samples require that the light REE enrichment which characterizes KREEP was established considerably before 3.85 AE. Within the limits imposed by model assumptions in the various radiometric systems, it is concluded that the extremely narrow spread of Sm-Nd model ages for these samples around 4.36 AE, and the compatibility of this age with that indicated by the U-Pb and Rb-Sr systems, indicate that the source of later KREEP volcanism was produced in the closing stages of an early global scale lunar differentiation episode.     

Channel incision,evolution and potential recovery in the Walla Walla and Tucannon River basins,northwestern USA

We evaluated controls on locations of channel incision, variation in channel evolution pathways and the time required to reconnect incised channels to their historical floodplains in the Walla Walla and Tucannon River basins, northwestern USA. Controls on incision locations are hierarchically nested. A first‐order geological control defines locations of channels prone to incision, and a second‐order control determines which of these channels are incised. Channels prone to incision are reaches with silt‐dominated valley fills, which have sediment source areas dominated by loess deposits and channel slopes less than 0·1(area)^?0·45. Among channels prone to incision, channels below a second slope–area threshold (slope = 0·15(area)^?0·8) did not incise. Once incised, channels follow two different evolution models. Small, deeply incised channels follow Model I, which is characterized by the absence of a significant widening phase following incision. Widening is limited by accumulation of bank failure deposits at the base of banks, which reduces lateral channel migration. Larger channels follow Model II, in which widening is followed by development of an inset floodplain and aggradation. In contrast to patterns observed elsewhere, we found the widest incised channels upstream of narrower reaches, which reflects a downstream decrease in bed load supply. Based on literature values of floodplain aggradation rates, we estimate recovery times for incised channels (the time required to reconnect to the historical floodplain) between 60 and 275 years. Restoration actions such as allowing modest beaver recolonization can decrease recovery time by 17–33 per cent. Published in 2007 by John Wiley & Sons, Ltd.