BASE LEVEL, AGGRADATION, AND GRADE LUNA B. LEOPOLD AND WILLIAM B. BULL Reprinted from PROCEEDINGSOF THE AMERICANPHILOSOPHICAL SOCIETY, Volume 123, No 2, April 1979 @DINGS of the ical Society Contents of olurne 123, Number 3 Variations in the Sun and Their Effects on Weather and Climate WALTERORR ROBERTS 151 Afro-American Biography : The Case of George Washington Williams JOI-IN HOPEFRANKLIN 160 Cultural and Social Background of the Rapid Modernization of Japan SANICI-IIROMIZUSHIMA 164 Base Level, Aggradation, and Grade LUNAB. LEOPOLD,WILLIAM B. BULL 168 Price for complete number $2.50 AMERICAN PHILOSOPHICAL SOCIETY INDEPENDENCESQUARE PHILADELPHIA, PA. 19106 BASE LEVEL, AGGRADATION, AND GRADE LUNA B. LEOPOLD Professor of Geology, University of California, Berkeley and WILLIAM B. BULL Professor of Geosciences, University of Arizona (Read April 20, 1978) GENERAL STATEMENT there is no long term change in climate or vegetation, and if the basin is not subjected to uplift or other JOHN WESLEYPOWELL introduced the term base eustatic movements, the river tends to attain an equ- level to specify the elevation below which a stream libriuni or graded coliclition characterized by a stable cannot downcut. Fluvial processes cease where a LGngitudinal profile neither raising or lowering in river flows into a large lake or tlie ocean, because the elevation. In each portion of river length that is in hydraulic gradient is reduced to zero and potential such equilibrium, the longitudinal profile of that por- energy is not further transfornied into kinetic energy. tion tends to be concave upward with gradient de- The ocean is regarded as a permanent hase level even creasing in the downstream direction. This flattening though sea level varies. of gradient is influenced by several factors but tht. Local base levels occur between the headwaters of principal cause is the downstream increase in dischargt. a stream and the ocean. For example, a resistant as tributaries enter. The concave upward longitudi- outcrop forms a local base lezwl because the stream nal stream profile is developed upstream from a base is unable to downcut through the outcrop as rapidly level or a local base level. This profile tends toward as through tlie materials upstream and downstream ‘such a shape that there is at each point along Its from it. Thus tlie resistant outcrop forms a relatively length a mutual adjustment among hydraulic factors stable reach to which the upstream portions of the which G. K. Gilbert referred to as “an equilibrium channel system flow until the outcrop is removed of action” (1880: 11. 107). by erosion or is buried by alluvium. Such local base William Morris Davis (1902 : 1). 107) proposed levels are regarded as being temporary when con- that tlie word “graded” be used specifically for the pared to the permanence of the oceans. Even more stream in which equilibrium is maintained and that temporary are the elevations of alluvial reaches of “degrading” and “aggrading” be restricted to cases streams, which also may be regarded as base levels. of the shifting equilibrium. J. Hoover Mackin ( 1948 : Each point along a stream, be it underlain by rock p. 478) stated that “ ‘degrading’ is downcuttitlg ap- or alluvium, is part of a continuum of elevations along proximately at grade” and that “ ‘aggrading’ is up- tlie streambed. Each streambed elevation partly de- building approximately at grade.” Mackin ( 1948 : p. termines the total relief (and therefore tlie slope of 471) defined grade as follows: the stream) for that part of the system upstream from the elevation. The graded stream is one in which, over a period of Elevations of points along a streambed may be years, slope is delicately adjusted to provide, with avail- able discharge and with prevailing channel characteristics, changed by the local base lezd processes of aggrada- just the velocity required for the transportation of the tion, degradation, and tectonic movements (B~ill. load supplied from the drainage basin. The graded 1973; Bull and McFadden, 1977). Variation of stream is a system in equilibrium; its diagnostic char- ocean or lake level is also a base level process. Al- acteristic is that any change in any of the controlling though base level processes nlay change streambed factors will cause a displacement of the equilibrium in a elevations along much of a stream system, base level direction that will tend to absorb the effect of the change. change generally is viewed as being a perturbation If the lower end of such an equilibrium profile is that occurs within a short reach, but which may affect raised or lowered by a rise or fall of the controlling tlie reaches upstream and downstream. Examples base level, it is generally supposed that given time would be the aggradation and degradatioli associated the whole profile will as a result tend to be raised with vertical fault rupture of a streambed, and the creation of natural or human-made dams. by deposition of sediment on the streambed or low- If, over a period of years, hydrologic conditions ered by erosion of the streambed. in other words in a river basin remain essentially constatlt, that is, by aggradation or degradation. VOL. 123, NO. 3, 19791 BASE LEVEL, AGGRADATION, AND GRADE 169 Mackin stated clearly how he conceived base level and the equilibrium profile to be related. A rise in base level is equivalent to the rising of a barrier across the path of the graded stream . the final readjusted profile will tend toward parallelism with the original profile . but because of secondary effects of aggradation . , precise parallelism will usually not be achieved (Mackin, 1948 : p. 496). Lowering of base level is, insofar as the response of the stream is con- cerned, essentially the same as the lowering of a barrier in its path. Downcutting must continue until slope is again completely adjusted to supply just the velocity required to transport all of the debris shed into the stream; as in the last case, and with the same qualifica- tions, the final adjusted profile will tend to parallel the original profile (Mackin, 1948: p. 498). This conclusion is inferred in part from the fact that in many river systems, stratigraphic and morpho- logic evidence shows that when a master stream aggraded, so also did entering tributaries. If a ter- race or level of an abandoned flood plain occurs along the valley sides of a master stream it is usual to see a similar terrace along the sides of tributary valleys and, at the junction of the valleys, the terraces are at the same elevation. FIG. 1. Aerial photograph of the Loop, an abandoned me- Similarly most tributaries enter the master stream ander, showing its relation to the present canyon of the at the elevation of the master stream and are said to San Juan River. be “graded to the master stream.” Only where a tributary is at its mouth traversing a resistant layer dation of valley fill alluvium during periods of un- of rock, or whose elevation’ was determined by some changing local base level. process operating in the past such as glacial action, By the analysis of field examples in which the sepa- does the tributary discharge over a waterfall into the ration of base level from basin changes can be made, master stream. Such “hanging valleys” are seen in we propose to draw some inferences that may clarify glaciated terrain and in deep gorges as in the Grand the relations between local base levels and base-level Canyon system. processes, and between a base-level change and the When a tributary meets the master stream at grade, longitudinal profile upstream. the usual case, it is not possible to say whether the effect of the master stream is felt far upstream or ALLUVIAL HISTORY OF A CHANNEL DURING merely a short distance. A rise or lowering of the SLOWLY LOWERING BASE LEVEL WHEN level of the master stream constitutes a base-level SEPARATED FROM THE MASTER STREAM change for the tributary. Therefore it cannot be The Loop of the San Juan River, Utah ascertained whether the observed change in the tribu- Preliininary remarks tary system was caused by change of base level or Rivers have a heritage but no beginning. Through by a combination of base-level change and concurrent geologic time even the first incipient channel system discharge-sediment relations affecting both tributary appearing on an emerging landmass changes in forin, and master stream simultaneously. The separate ef- drainage network, and gradient as relief increases. fect of such base-level changes cannot be isolated. There is continual modification, but at any stage the existing system is different from that which existed PURPOSE AND SCOPE at a previous state, but has been influenced by that The influence of local base level and the effects of earlier stage. a change of base level can be separated from the in- Therefore there is a special interest attached to any fluence of hydrologic and geomorphic processes within fluvial system that, owing to some geologic accident, the basin under certain conditions. has a definite and known origin. A singular case of These include the following situations : 1) observed this is an abandoned meander loop that becomes iso- changes in the longitudinal profile by aggradation or lated from the river of which it was once a part. degradation when a local base level is lowered or Three examples exist in the canyon country in the raised while the basin conditions remain unchanged. middle reaches of the Colorado River in Utah; the 2) observed changes in aggradation and/or degra- Rincon of the main Colorado River, the Gooseneck in 170 LEOPOLD AND BULL [PROC. AMER. PHIL. SOC. FIG.2. Topographic map of the Loop.