Number Of First-order Streams Research Articles

Stream networks and knickpoints in the Sanjiangyuan Region

Stream networks and knickpoints in the Sanjiangyuan RegionThe source area of the Lancang (Mekong), Yangtze and Yellow rivers is named in Chinese Sanjiangyuan (source of three rivers). Geographical characteristics of these rivers, and various rivers at the margin of the Qinghai-Tibetan Plateau (Jialing, Minjiang, Dadu, Yalong and Jinsha) are summarized from field investigations along with digital elevation model (DEM) analyses and satellite images. Uplift of the Qinghai-Tibetan Plateau has resulted in an asymmetrical distribution of tributaries for stream networks at the margins of the plateau. Almost all tributaries join the trunk stream from the northwest. Three types of drainage network are evident: plume, nervation and dendritic. In general, plume networks have a large number of first order streams. Nervation networks have a main stream and parallel tributaries. Dentritic networks are characrterized by continuous bifurcation and have a branch-like appearance. Most stream networks in the Sanjiangyuan region are of the nervation type. Several large knickpoints are evident along the longitudinal profiles of the Yellow and Yangtze Rivers. Channel aggradation immediately upstream of these knickpoints marks a transition in river processes from vertical bed evolution (i.e. incision) to horizontal channel adjustment (and associated braided and anabranching channels).

Réponse des bassins versants à l'activité tectonique : l'exemple de la terminaison orientale de la chaîne pyrénéenne. Approche morphotectonique / Drainage basins response to active tectonics: example from Eastern Pyrenees. Morphotectonic approach

Extended Abstract The aim of this paper is to demonstrate the validity of morphométrie analysis in an area of low seismo-tectonic activity. There, drainage divide responds to uplift modifying their morphologic and sedimentologic parameters, such as drainage density, drainage divide size, hypsometric integral value. Tectonic and geomorphic investigations are focused on the relationship between Plio-Pleistocene deformation and landform ruptures using quantitative geomorphology. Morphométrie and geomorphic analysis of drainage basins can be correlated with tectonic input. Correlation research between channel network parameters (such as slope, channel length and drainage density), or basin parameters (area, relief and hypsometry) and uplift is tested. This method is applied to the Tet drainage basin (1500 km2), a Mediterranean coastal river, (eastern Pyrenees, South France). This river was developed during the Neogene time along a major regional fault, NE-SW direction, the Têt fault. This fault is still active (Souriau and Pauchet, 1998). Numerous drainage anomalies, such as antecedence, dissymetry and avulsion, lead us to expect drainage responses to active tectonics. First, Horton's laws are tested. Confluence and length ratio are homogenous in the Tet drainage divide. They reflect the organisation of the drainage network at the regional scale and are not related to local tectonic variations, as previously shown by Kirschner (1993). First order drainage frequency, defined as the number of first order streams per square km, reveals a North - South drainage basin structural asymmetry. The southern part of the drainage basin is excessively dissected. This opposition is confirmed by basin parameter analysis. The sizes of the third and fourth order basins show antagonistic responses at the periphery of the Tet basin. In the southern portion of the basin, the median size of the. third and fourth order basins is 50% greater than their northern counterparts. The same observation can be made about the hypsometric curve. Moreover, this parameter points out the presence of an est - west gradient. On the southern margin of the Têt basin, hypsometric integrals decrease from 0,36 to 0,63. These values are highly correlated with tectonic uplift as deduced from scarp fault morphology, using height, slope and complicity ratios of the vounger spure generation. Morphometric results are compared with field data. The calculation of deformation rates by two different methods shows a linkage between them. During the Pliocene, uplift rates varied from 0.2 mm/year in the eastern part of the fault to 1 mm/year in the western part. The morphostructural organisation of the eastern part of the Pyrenees shows evidence of strike-slip faulting, related to a meridian compressional stress regime. It induced left-lateral faulting on the Tet fault and a propagation of deformation. As a result, the inversion of stress regime in the Pliocene, previously pointed out by other studies (Grellet et al., 1993), is confirmed by a geomorphological approach.

Hydraulics and basin morphometry of the largest flash floods in the conterminous United States

The maximum rainfall-runoff floods measured by indirect methods in small basins (0.39–370 km 2) in the conterminous United States are examined. This analysis identified twelve floods that were the largest ever measured. These floods all occurred in semiarid to arid areas. For eleven of the twelve largest rainfall-runoff floods measured in small basins by the slope-area method, values of hydraulic depth; hydraulic radius; width-depth ratio; n value; mean velocity; velocity-head coefficient; Froude number; water-surface, energy, and channel slopes; shear stress; and unit stream power are tabulated. Estimated composite n values weighted by subsection conveyance range from 0.028 to 0.048 with a mean of 0.038. Mean velocities ranged from 3.47 to 9.97 m s −1. Froude numbers ranged from 0.81 to 2.49, with 9 of 12 floods having values greater than 1.00. Water-surface, energy, and channel slopes vary considerably for each flood. Energy slope always was less than water-surface slope by values of 1–104%. Channel slope was greater than energy slope in eight floods. Shear stresses ranged between 61 and 855 N m −2, and unit stream power from 212 to 8131 w m −1. Floods in these small basins produced shear stresses and unit stream powers several hundred times greater than floods in large rivers. Floods on other small streams, with smaller unit discharges, produced greater shear stresses and stream powers. This indicates that the force of a flood is controlled by the depth-slope product, not absolute discharge. In the twelve watersheds studied, basin relief ranged from 165 to 1280 m, elongation ratios ranged from 0.55 to 0.80, the number of first-order streams (basin magnitude) ranged from 10 to 4297, drainage density ranged from 4.1 to 10.9 km km −2, basin slope ranged from 0.0043 to 0.2486, relief ratio ranged from 0.0097 to 0.34, ruggedness number ranged from 0.69 to 7.17, and first-order channel frequency ranged from 5.1 to 38.6 km −2. Elongation ratios were larger, and drainage density and first-order channel frequency lower, than other small flash-flood prone basins in the United States. These twelve basins have neither the most favorable morphometric characteristics which contribute to flash-flood peaks, nor did the storms causing these flash floods have the greatest short-duration intensities. Maximum flood peaks originate from an optimal combination of basin morphology and physiography, and storm intensity.

Land management, geological and soil effects on streamwater chemistry in upland mid-Wales

Streamwater chemical data are presented to show that the difference in chemistry between a number of first-order streams can be interpreted in terms of bedrock, soil and land management influences. A change in lithology within a geochemically uniform bedrock had little effect on streamwater chemistry. Small-scale calcite mineralization, however, coincided with significant increases in pH and the concentrations of calcium and bicarbonate; other solutes were generally unaffected. Land improvement resulted in increased concentrations of several solutes; some reflecting addition as fertilizer and others the effects of cultivation. Differences in the amount of peat within the catchments and hence the proportion of water derived from peat also influenced the pH and the concentrations of calcium and bicarbonate present in the streamwater.

Hydrogeomorphic methods for the regional evaluation of flood hazards

The “upstream” approach to flood hazard evaluation involves the estimation of hydrologic response in small drainage basins. This study demonstrates the application of geomorphology to such studies in a region of unusually intense flooding in central Texas. One approach to flood hazard evaluation in this area is a parametric model relating flood hydrograph characteristics to quantitative geomorphic properties of the drainage basins. A preliminary model uses multiple regression techniques to predict potential peak flood discharge from basin magnitude, drainage density, and ruggedness number. After mapping small catchment networks (4 to 20 km2) from remote sensing imagery, input data for the model are generated by network digitization and analysis by a computer-assisted routine of watershed analysis. The study evaluated the network resolution capabilities of the following data formats: (1) large-scale (1:24,000) topographic maps, employing Strahler's “method of v's”, (2) low altitude black-and-white aerial photography (1:13,000 and 1:20,000 scales), (3) NASA-generated aerial infrared photography at scales ranging from 1:48,000 to 1:123,000, and (4) Skylab Earth Resources Experiment Package S-190A and S-190B sensors (1:750,000 and 1:500,000 respectively). Measured as the number of first order streams or as the total channel length identified in small drainage areas, resolution is strongly dependent on basin relief. High-density basins on the Edwards Plateau were poorly depicted on orbital imagery. However, the orbital network definition of low-relief basins on the inner Texas Coastal Plain is nearly as accurate as results from large-scale topographic maps. Geomorphic methods are also useful for flood hazard zonation in “downstream” flood plain areas. Studies of the Colorado River valley near Austin, Texas, easily distinguished infrequent (100- to 500-year recurrence interval), intermediate (10- to 30-year), and frequent (1- to 4-year) hazard zones. These mapping techniques are especially applicable to the rapid regional evaluation of flood hazards in areas for which there is a lack of time and money to generate more accurate engineering-hydraulic flood hazard maps.

MAP SCALE EFFECT ON THE STREAM ORDER ANALYSIS

It can readily be imagined that the result of morphometry of a given region will be different according to the scale of topographical map used in morphometry. Stream order analysis is attempted on the drainage basin of the Oguri River, a tributary of the Tama River. Four kinds of topographical maps of different scale (1:3, 000, 1:10, 000, 1:25, 000, 1:50, 000) are used. Drainage system maps are drawn from these maps respectively. The Strahler system of stream ordering is adopted and the whole basin is subdivided into 26 subbasins. The drainage networks of these subbasins are shown in Figures 1, 2, 3 and 4. The result of the stream order analysis obtained from each map system is tabulated on Table 1. Comparison of these figures enables us to recognize the effect of map scale especially at the head of stream network where the number of first order streams decreases with the de crease in map scale. As can be seen from Fig. 7, plots of the logarithms of the number of first order streams against denominator of map scale show a linear relationship between them. The regression equation for this line is expressed approximately in the form of LogN=LogK-LogS where K denotes the vertical intercept of the line. Although the Strahler's stream number assigned to a particular stream segment depend on the scale of topographical map used in ordering the stream network, the Horton-Strahler straight line for the stream number is found to be independent of map scale as shown in Fig. 8. The result obtained supports the Yangs' conclusion that the Horton-Strahler relationship obtained from topographical maps of different scales for the same stream system have the same bifurcation ratio of stream number. Concerning stream length, most of the stream net works of subbasins do not conform to the law of stream length. Sometimes in some basins, the higher order stream segments are shorter than they should be. It may be that, in this case, lack of agreement with the Horton's law is a result of the Strahler method of ordering. Comparison of actual stream system and that of the largest scale map suggest that stream system obtained from larger scale maps includes intermittent streams, which is considered to be less effective as an erosive agents. Therefore, maps of larger scales are not necessarily indispensable for stream order analysis.

Dependence of stream link lengths and drainage areas on stream order

It has been suggested in the literature that the lengths of interior links (distances along river course between subsequent junctions) in a given river network are independent random variables drawn from the same population. However, the validity of this basic hypothesis with relation to nature had never been investigated. Thus a test was made to find the exact relationship between the stream lengths and the corresponding stream orders by comparing the theoretical length ratios calculated, assuming all link lengths to be equal, and the natural length ratios for rivers with the same number of first order streams. The results show that the link length always has a tendency to increase geometrically with the Strahler order. This leads to the conclusion that a law of link lengths exists that is similar to the well‐known laws of stream orders, stream lengths, and areas. A similar analysis with regard to the areas drained by each link in a river network shows that these areas are constant.

Statistical Law of Stream Numbers

The statistical nature and remarkable generality of Horton's law of stream numbers suggest the speculation that the law of stream numbers arises from the statistics of a large number of merging stream in somewhat the same fashion that the law of perfect gases arises from the statistics of a large number of colliding gas molecules. The fact that networks with the same number of first-order Strahler streams are comparable in topological complexity suggests equating randomly merging stream channels with a topologically random population of channel networks, defined as a population within which all topologically distinct networks with given number of first-order streams are equally likely. In a topologically random population the most probable networks approximately obey Horton's law but exhibit certain systematic deviations. For networks with given number of first-order streams, the most probable network order is that which makes the geometric mean bifurcation ratio closest to 4....