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Increased availability and resolution of remotely-sensed (RS) imagery of Earth's surface has greatly enhanced the precision, spatial extent, and temporal frequency at which we can analyze river channel migration and width changes. Despite a body of research identifying and quantifying sources of uncertainty inherent in such data, no framework has emerged to comprehensively quantify and handle uncertainty. Herein, we summarize and evaluate present best practices, test new approaches to quantify and handle uncertainty, and provide recommendations for future work using remotely-sensed measurements of river migration and width changes. While our research focuses on river systems, the principles and approaches are applicable to research delineating boundaries or using boundaries to measure changes: glacier retreat or advance, erosion or deposition along coastlines and lakeshores, changes in wetland extent, expansion or contraction of vegetation (e.g., deforestation), cliff retreat, sea level rise due to climate change, change in aeolian depositional systems, and anthropogenic/political boundary disputes. From our results, the following conclusions and recommendations arise: 1. Planform change measurements should span spatial intervals larger than coherent units of adjustment to avoid spatial autocorrelation. 2. Uncertainty in manual riverbank delineations is dominated by arbitrary user inconsistency rather than poor image quality (i.e., resolution, colour versus grayscale, year of acquisition) or environmental conditions (i.e., shadows and vegetation cover). 3. We recommend that digitizations follow the vegetated boundary that best approximates bankfull width, whenever possible, to avoid inconsistency along ambiguous reaches. 4. Using a spatially variable level of error detection (LoD) threshold improves the quantity and quality of retained measurements relative to a uniform LoD. 5. After applying a LoD threshold, we recommend first using expert discretion to manually classify any ‘nondetect’ measurements that qualify as ‘significant’ measurements of zero (i.e., no change actually occurred). 6. Subsequently, three methods may be used for handling the remaining nondetects; Kaplan-Meier (KM) and Maximum Likelihood Estimators (MLE). The specific approach chosen for handling nondetects is contingent upon each case, but can be guided and informed by descriptions and assumptions of each method, references to external resources, and results of our river-focused analyses. 7. Finally, we encourage a focus on improving the simplicity, generalizability, and open-source opportunities of tools and packages used for calculating river planform change and spatially variable uncertainty, thereby enabling a common platform to measure and compare results.

A paucity of research exists regarding the millennial-scale response of inland alluvial streams to abrupt base-level fall. Studies of modern systems indicate that, over short time scales, the response is a diffusion-like process of upstream-propagating incision. In contrast, evidence from the lower Chippewa River (LCR), located in the upper Midwest of the USA, suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion, resulting in incision that is prolonged and episodic. During the Last Glacial Maximum, the LCR drained the Chippewa Lobe of the Laurentide Ice Sheet to the glacial upper Mississippi River (UMR). As a meltwater stream, it aggraded and filled its valley with glacial outwash, as did its largest tributaries, which were also meltwater streams. Its nonglacial tributaries aggraded, too, filling their valleys with locally derived sediment. During deglaciation, the UMR incised at least twice, abruptly lowering the LCR's base level — ~ 15 m at 16 ka or earlier and an additional 40 m at ca. 13.4 ka. Each of these base-level falls initiated incision of the LCR, led by upstream migrating knickpoints. The propagation of incision has, however, been a lengthy process. The optically stimulated luminescence (OSL) ages of terrace alluvium indicate that, by 13.5 ka, incision had advanced up the LCR only 15 km, and by 9 ka, only 55 km. The process has also been episodic, resulting in the formation of fill-cut terraces (inferred from GPR surveys and exposures of terrace alluvium) that are younger and more numerous in the upstream direction. Autogenic increases in sediment load and autogenic bed armoring, the result of periodic tributary-stream rejuvenation and preferential winnowing of fines by the incising river, may have periodically caused knickpoint migration and incision to slow and possibly stop, allowing lateral erosion and floodplain formation to dominate. A decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred. Minimal floodplain development along the upper section of the present-day LCR, along with the channel morphology of an unstable wandering gravel-bed river immediately downstream from it, suggest that the river is still responding to the base-level falls that happened many millennia ago. The autogenic controls on the LCR's response to UMR incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the Late Wisconsinan, making the LCR a prime example of a former proglacial river that remains a paraglacial fluvial system.

The Salt River and Verde River watersheds provide downstream metropolitan Phoenix, Arizona, USA with much of its water supply, and this paper explains how these rivers integrated in an extensional tectonic setting. Near the end of the Pliocene, segments of the proto-Salt and proto-Verde watersheds of central Arizona consisted of local drainage networks supplying water and sediment into internally drained basins, including depressions occupied by late Pliocene natural lakes occupying the Verde Valley and Tonto basins. A key location, the lower Verde River valley (LVRV), is where the modern-day drainages of the Salt and Verde now meet downstream of these Pliocene lakes. At the time of the Nomlaki tuff deposition ~3.3 Ma, a condition of sediment overfill existed in the LVRV, although there was no exoreic drainage and a playa was still maintained. A fanglomerate unit, named here the Rolls formation, spilled over a bedrock sill and an alluvial-fan ramp transported sediment into the Higley Basin that underlies the eastern part of metropolitan Phoenix. Lithologies of preserved remnants of this Pliocene alluvial-fan system match well cuttings of buried sediment in the Higley Basin with a cosmogenic burial isochron age of 3.90 ± 0.70 Ma. Based on cosmogenic burial isochron ages, ancestral Salt River gravels started depositing on top of this fan ramp between 2.8 and 2.2 Ma. Deposition of Salt and Verde river gravels in the Higley Basin continued for ~2 million years and eventually led to an aggradational piracy event that overtopped a bedrock ridge immediately east of Phoenix's Sky Harbor Airport. A cosmogenic burial age of 460 ± 23 ka is a rough maximum age for this river avulsion that relocated the Salt River into the Luke Basin that underlies western metropolitan Phoenix. Available chronometric data are not precise enough to determine whether the Salt River or Gila River integrated first. All of the exoreic rivers of western North America's Basin and Range Province — the lower Colorado, Gila, Rio Grande, Salt, and Verde rivers — employed lake overflow to integrate across half-graben, graben, rift and supradetachment tectonic settings.

The Salt and Verde rivers, Sonoran Desert, USA, integrated multiple endorheic extensional basins near the start of the Quaternary. Integration began via spillover on both rivers. Spillover resulted in headward erosion in upstream basins, leading to basin-wide incision and excavation of large volumes of sediment transported into downstream basins. As a result, the Verde River aggraded to what is now the highest terrace in the lower Verde River valley (LVRV), the Lousley Hills terrace. Evidence suggests deposition of the Lousley Hills terrace fill led to aggradation and backfilling of adjacent pediments. Ancestral Salt River Deposits (ASRD) of the combined Salt and Verde rivers deposited an extensive alluvial fan in the Higley Basin, raising local base level for over 2 million years. Eventually, this led to aggradational piracy of the Salt River (~0.46 Ma) that integrated the Higley and Luke basins. Consequently, this shortened and steepened the Salt River, resulting in ~30 m of aggradation downstream and headward incision upstream. Headward incision abandoned the ASRD and created the Sawik stream terrace. Piedmont pediment and alluvial fan systems that were formerly adjusted to the ASRD incised in response. Since abandonment, the ASRD floodplain has accumulated a sheet of silt and fine sand from aeolian and local fluvial processes. Sedimentary, metamorphic, and granitic pediments that developed in slowly aggrading endorheic basins display evidence of response to base-level adjustments resulting from drainage integration processes. Classic ballena landforms (eroding alluvial fans) began to form in the LVRV only after drainage integration — providing the first known maximum age for the ballena form. Drainage integration of the Salt and Verde rivers clearly demonstrates the impact of base-level fluctuations on basin-scale geomorphology. However, integration led to very different geomorphic responses in different extensional basins, revealing the difficulty of a one-size-fits-all conceptual model of geomorphic response drainage integration.

The top-down drainage basin integration and drainage network evolutionary processes of lake overflow and aggradational spillover (or aggradational piracy), collectively referred to here as spillover, are undergoing a revitalization in the literature to explain aspects of drainage basin evolution and transverse drainage development across the globe. Spillover processes are commonly unidentified or misidentified as other processes (e.g., piracy/capture via headward erosion, superimposition, antecedence). Commonly invoked arguments for stream piracy associated with “headward erosion” are discussed in the context of contemporary research highlighting the inefficiency of this process. The term “headward erosion” in this context is often misconstrued and confused with headward propagating knickpoints in preexisting drainages that are actively evolving. This term is, therefore, misused when discussing basin integration and the establishment of new transverse drainages. We propose the term “drainage-head erosion” to rectify this and recommend that “headward erosion” only be used when referring to real headward erosion when a preexisting stream has headward propagating knickpoints or a knickzone. Recent investigations have pointed to an engrained paradigm and pedagogical bias as a possible culprit for the lack of recognition of lake overflow and aggradational spillover. Despite this, spillover is common in a variety of terrestrial and extraterrestrial settings, including geomorphic and geographic settings ranging from post-glacial drainage reorganization to drainage integration processes in extensional tectonic terrains. The focus of this manuscript is to provide a summary of these processes as presented in the scientific literature and to highlight the revitalization of spillover in present research with focus on two prominent geomorphic settings where spillover processes commonly occur: proglacial environments and extensional terrains like those of the southwestern United States.

This paper provides support for the concept of geomorphic process domains developed by Montgomery (1999) by linking geomorphic processes to ecological variations seen in the Pima arid granitic watershed of the Sonoran Desert, Phoenix, Arizona. Closer joint spacing shows a statistically significant correlation with lower percentages of mineral grain attachment as measured by digital image processing of backscattered electron microscope imagery. Lower mineral grain attachment leads to more frequent spalling of rock surfaces, as measured by varnish microlamination (VML) ages of the last spalling event. In contrast, more distant joint spacing leads to in situ 10Be erosion rates of 3.4–8.5 mm/ka and the emergence of low domes and kopje granitic landforms; these low domes also serve as knickpoints along ephemeral washes. Distant jointing thus plays a key role in generating the bare bedrock surfaces that funnel limited precipitation to bedrock margins — enhancing the canopy cover of perennial plants next to the bare bedrock. Joint-influenced geomorphic processes at Pima Wash generate four distinct process domains: (PD1) armored drainage divides; (PD2) slopes with different granite landforms; (PD3) mid- and upper basin channels that mix knickzones, strath floodplains, and sandy alluvial sections; and (PD4) the main ephemeral channel transitioning to the piedmont. Distant jointing promotes bedrock exposure and rock armoring along drainage divides in PD1 that then concentrates runoff and promotes perennial plant growth. More distant joint spacing on slopes in PD2 promotes exposure of granitic bedrock forms that shed overland flow to their margin and promotes flora and fauna growths along the margins of low granitic domes and kopjes. Similarly, wider joint spacing along ephemeral washes in PD3 leads to knickpoints, which in turn act to concentrate moisture immediately downstream. The stream terraces in PD4 influence the ecology through xeric desert pavements on terrace treads and roofs for coyotes (Canis latrans) and gray fox (Urocyon Cinereoargenteus) dens on terrace scarps via stage 3 pedogenic carbonate. These four process domains occur in six other randomly selected granitic watersheds with drainage areas