Abstract

Spatial and temporal variations in watershed characteristics provide insight into the controls on discharge, sediment transport, and channel morphology for the Río Panuco, Mexico, the sixth largest (98,227 km²) river flowing into the Gulf of Mexico. Although there has been considerable study of Gulf Coastal Plain river systems within the United States, there have been very few studies of rivers along the Mexican Gulf Coastal Plain. Data from fieldwork and from Mexican agencies suggest three primary findings. First, stream flow and suspended sediment data reflect the seasonal timing and strengthening of summer trade winds and tropical storm systems. Secondly, a profile of the change in sediment size developed from samples of channel lag deposits along the Lower Moctezuma and Río Panuco represents an exception to the oft-reported exponential curve of downstream sediment trends. Finally, the presence of the Panuco High anticline represents an inherited control on contemporary channel and hydrologic processes.

Keywords: Río Pánuco, Mexico, fluvial geomorphology

Introduction

Large river systems are influenced by a variety of climatic and geologic controls within their watersheds (Schumm and Winkley 1994). Therefore, an understanding of the behavior of large river systems requires knowledge of the spatial variation in drainage basin controls. Although the large river systems of the United States Gulf Coastal Plain have been extensively studied, with the exception of West et al. (1969), the large river systems of the Mexican Gulf Coastal Plain have not been examined in detail. This creates a significant gap in knowledge concerning Gulf Coastal Plain geomorphology, which is biased toward the United States side of the Río Grande. This study offers a preliminary examination of the seasonal and spatial variability of watershed controls on the hydrology, sediment, and channel characteristics of the Río Pánuco, a large (98,227 km²) Mexican Gulf Coastal Plain river system (Figure 1).

Natural Setting

Physiography

The Río Pánuco forms at the confluence of the Río Moctezuma and Río Tamuin, large river systems that drain the humid eastern flanks of the Sierra Madre Oriental. Downstream of the confluence, the river flows 172.5 km to Tampico, Tamaulipas, where it discharges into the Gulf of Mexico (Figure 2). The watershed drains three physiographic regions, the Central Plateau, Sierra Madre Oriental, and the Gulf Coastal Plain. While the United States Gulf Coastal Plain is generally considered one of the least complicated geological provinces in North America (Walker and Coleman 1987), the Mexican Gulf Coastal Plain is more[end p. 61]

complex. The Mexican Gulf Coastal Plain extends 90 km from the coast at Tampico and terminates sharply with the escarpments of the Sierra Madre Oriental. The Coastal Plain in the vicinity of Tampico consists of coastal dipping Tertiary and Quaternary deposits of fluvial, marine, and volcanic origin (Muir 1936; Grubb and Carillo 1988; de Cserna 1989). A further distinction from the United States Gulf Coastal Plain is the presence of large structural controls, including the Pánuco High, a southerly plunging anticline that intersects the Río Panuco near Ciudad Panuco (Trager 1926) (Figure 3).

Climate

The Pánuco basin is influenced by three climatic regions. Annual precipitation averages only 300 mm where the headwaters of the basin drain the arid central plateau, west of the Sierra Madre Oriental (Köppen Bwh). The majority of the basin straddles the boundary between the subtropical, semiarid climatic zone to the north (Köppen Bsh) and the subtropical, subhumid climatic zone to the south (Köppen Aw), with average annual precipitation ranging from 800 to 3000 mm, respectively. Unlike mid-latitude regions that are heavily influenced by cyclonic (frontal) precipitation mechanisms, precipitation patterns along the Mexican Gulf Coastal Plain are predominantly controlled by two mechanisms and reflect a strong seasonal pattern. From late-June through August, easterly trade winds strengthen and provide a consistent source of moisture. From August through October, tropical storm systems may originate either in the Atlantic or southeast of Tampico within the Gulf of Mexico or Caribbean, and deliver large amounts of precipitation to eastern Mexico (Figure 4). Combined with the orographic effect from the steep Sierra Madre Orientals, large amounts of rainfall occur along the windward (eastern) side of the Sierra Madre from late-June to October, but it remains fairly dry from November through mid-June. A less important source of precipitation is due to the occurrence of nortes (mid-latitude cyclones), which migrate south from the United States providing precipitation for several days during the winter (West et al. 1969).

Discharge and Suspended Sediment

Discharge

Although tropical river systems transport the majority of the world's surface runoff (Knighton 1998), few studies have examined fluvial processes within this setting. The majority of studies have examined smaller river systems in temperate mid-latitude climates that are dominated by cyclonic precipitation. Figure 5 displays a typical annual hydrograph (1990) for the Río Pánuco at Las Adjuntas, Río Moctezuma at El Olivo, and Río Tamuin at Tamuin. The average daily discharge for the Río Pánuco at Las Adjuntas, just downstream of the confluence of the Moctezuma and Tamuin, was 683 cubic meters per second (cms). Data for the Moctezuma and Tamuin were collected at the lowermost gauging stations, just upstream of the confluence and formation of the Pánuco (Figure 3). With an average annual discharge of 349 cms, the Río Moctezuma is the major contributor of discharge to the Panuco. The Río Tamuin also contributes a substantial amount of discharge to the Panuco and has an average annual discharge of 270 cms.

The annual discharge pattern for the three rivers displays distinctive seasonality. The peak discharge occurs in late summer and early fall and then substantially decreases, remaining fairly low through June. This can be attributed to the combination of the two dominant precipitation mechanisms. The easterly trade winds reach their northern limit at this latitude, near the Tropic of Cancer, but substantially weaken after the summer solstice (West et al. 1969). In late summer and early fall tropical storms may form in the Gulf, Caribbean, or Atlantic and migrate to the western Gulf of Mexico. For example, a tropical depression stalled over eastern Mexico during September and October 1999 (Figure 4) and provided a record amount of precipitation that resulted in extreme flooding. While the flooding was more severe in southeastern Mexico, it was also a large event in the Pánuco watershed, resulting in the first overbank flood event in the Lower Moctezuma, Tamuin, and Panuco since 1993.

Sediment

Figure 6 displays the 1990 monthly suspended sediment discharge for the Río Pánuco at Los Adjuntas. Although this data set does not take into account the bed load, this is not likely to be a significant omission, as the bed load is typically estimated at ten percent of a river's total sediment load (Richards 1982). The pattern reflects the strong seasonal flow regime of the Río Pánuco, with suspended sediment discharge abruptly increasing with discharge in July and peaking with the late summer flood, transporting [end p. 64] 1,087.0 x 103 tons during the month of August. Suspended sediment discharge declines steadily in September and October, after the high discharge in August, and remains uniformly low from November through June, averaging only 26 x 103 tons during these months. The significance of the seasonal precipitation regime is that it sets up a seasonal disparity between the amount of work (sediment transport) accomplished by the fluvial system and time. For example, of the total suspended sediment transported during 1990, 93 percent of the sediment load was transported in only 33 percent of the time, from July to October.

Table 1 shows the average annual suspended sediment discharge for large rivers of the Mexican Gulf Coastal Plain. Although the years of data do not all coincide, it serves as a relative comparison between the major basins. The Río Pánuco at Los Adjuntas annually transports an average of 5,185 x 103 tons of sediment. However, there are considerable differences in the amount of sediment transported between the Río Moctezuma and Río Tamuin. The average annual sediment load of the Río Tamuin at Tamuin is 2,167. x 103 tons, while the Moctezuma at El Olivo transports an average of 6,026 x 103 tons of sediment per year. A variety of factors can influence sediment discharge (scale, geology, climate, vegetation, and human impacts), complicating comparisons between basins (Walling and Webb 1983). However, differences in sediment volumes transported by the Tamuin and Moctezuma are likely due to differences in geology, which is strongly suggested by difference in water color. The jade-colored waters of the Río Tamuin and its tributaries reflect the karstic nature of the basin. Much of the water supplied to the Tamuin derives from springs emitting from the Cretaceous limestone that comprises the eastern flanks of the Sierra Madre Oriental. While the sediment load was not directly measured, the lack of silt and clay (wash load) transported in the water column of the Río Tamuin permitted observation of coarser sands and gravels saltating along the channel bed.

In comparison, the geology underlying the eastern tributary basins of the Río Moctezuma is much different. The weathered and hilly terrain consists mainly of friable Tertiary (Oligocene) shale, interbedded with poorly developed sandstone (Muir 1936). The Moctezuma's café colored water reflects the addition of these easily eroded deposits. This is consistent with the study by West et al. (1969) of the Río Mezcalapa and Río Usumacinta of Tabasco. For comparison, the Usumacinta (63,804 km²) transports an average annual sediment load of 6,257 x 103 tons, while the Mezcalapa (36,566 km²) transports an average annual sediment load of 24,134 x 103 tons. In spite of the Mezcalapa being considerably smaller than the Usumacinta, the large sediment loads are derived from highly erodable Tertiary shale and sandstone and deeply weathered granite. The Cretaceous limestone of the Usumacinta supplies a higher dissolved load, but less clastic sediment (West et al. 1969). Finally, although the Río Grande has an immense drainage basin (453,250 km²), the sediment load (792 x 103 tons) is much lower than the humid basins to the south, which is likely due to arid climates producing less sediment (Walling and Webb 1983), and storage of sediment within the floodplain and reservoirs (Hudson and Mossa 1997).

Additional insight into the relative importance of specific drainage basins as suppliers of sediment can be considered by examining the downstream trend in sediment size. To get a sense of downstream changes in sediment size within the Pánuco system, sediments deposited from a July 1999 high-stage event were sampled at seven sites along the Moctezuma and Panuco. All sediments were sampled at the high water mark (not an overbank event), which was easily identified by the presence of a debris line and recently buried vegetation. To insure that deposits had not been biased by flow obstruction, all sampling occurred upstream of bridges, structures, or large trees. Particle-size analysis was performed using standard hydrometer and wet sieve methods (see Gee and Bauder 1986).The percentage of sand within each sample (percent of sample having a particle size larger than 0.0625 mm) was used as an index to interpret downstream changes in particle size.

Figure 7 shows the downstream trend in percent sand for sediment samples of the Lower Moctezuma and Pánuco. While the pattern reflects a typical downstream fining in grain size, the trend is irregular, suggesting spatial variability in controls. The sample near Tampacan (322.5 km) is representative of the Río Moctezuma as it exits the mountains and flows within a narrow floodplain. The river transports a considerable amount of coarse sediment, as 82 percent of the sample was sand. However, the next sample, at Tanquien de Escobedo (282.5 km), shows a large reduction in the percentage of sand, to 43 percent, which is likely due to hydraulic sorting. This site is 40 km downstream from the upstream sample, where the Moctezuma flows within a broad floodplain setting with a lower gradient favorable for deposition. The sample at El Higo (225 km), 57.5 km downstream of Tanquien de Escobedo, reveals a slight increase in the percent of sand, from 42 to 49 percent sand. The sediment was sampled one km downstream from the confluence with a large tributary, the Río Tempoal (6,651 km²), which disrupt the smooth downward curve by producing a spike in the trend (Ferguson 1987).

The trend downstream of the confluence of the Moctezuma with the Tamuin suggests an initial coarsening of sediment, followed by a rapid downstream fining of sediment. In comparison to the sample at El Higo, which had 49 percent sand, the proportion of sand at 117.5 km increased to 66 percent. The considerable increase in sand is surprising since it was sampled 107.5 km downstream of El Higo. However, there are a couple of factors that may explain this pattern. The higher percentage of sand transported by the Pánuco may be due to an influx of coarser sediments from the Río Tamuin. In comparison to the Moctezuma, the sediment load of the Tamuin has a higher proportion of coarser sediments, transporting much less silt and clay. Moreover, in several upstream reaches the Pánuco is reworking older Tertiary deposits that are added to its sediment load. Sediments sampled at 107.5, 90, and 70 km reveal a rapid decline in sand. The reduction in sand probably represents storage of sediment on the well-developed sequence of point bars along the Río Pánuco, as well as a lack of sediment inputs from tributaries.

Controls on Channel Pattern and Flooding

Large meandering alluvial rivers are dynamic entities, and adjust their channel [end p. 66] morphology toward an equilibrium profile in response to variations in controls (Schumm and Winkley 1994). However, in the absence of inputs of discharge and sediment, such as in the Río Pánuco, older valley deposits or neotectonics may become dominant, representing inherited controls that influence the distribution of energy loss along the river (Hooke 1995; Knighton 1998). A fundamental way in which meandering rivers adjust to changes in channel controls is through their planform morphology, which represents an adjustment in channel resistance (Harbor 1998). An approach commonly employed to evaluate spatial variability in planform morphology is the index of sinuosity, which is the ratio of the length of a channel segment to a valley segment. Straight channels have a sinuosity of 1.0, with higher values indicating increasing sinuosity. Maximum values of sinuosity rarely exceed 4.0. A meandering river is generally considered to have a sinuosity greater than 1.5 (Leopold et al. 1964), although this threshold has been criticized as being arbitrarily defined (Knighton 1998).

The Río Pánuco has a sinuosity of 1.85 over its 172.5-km channel. However, there is considerable spatial variability in sinuosity, allowing the channel to be subdivided into three distinctive segments (Figure 8).The upper Pánuco extends from the confluence of the Moctezuma and Tamuin to just upstream of Ciudad Pánuco, a distance of 72.5-km, and it has a sinuosity of 1.79. Several channel reaches within this section are in contact with older deposits, which presents the channel with heterogeneous bank material and spatial variability in channel resistance. This produces non-uniform erosion rates and an irregular planform morphology, which deviates from the hypothesized symmetrical sinusoidal meander bends found in laboratory models and where rivers flow through wide alluvial valleys with homogeneous deposits (Friedkin 1945; Leopold et al. 1964).

Sinuosity increases by 57 percent, from 1.79 to 2.81, in the middle Panuco, which extends from 100 km to 30 km above the mouth of the river. The valley for this section makes an abrupt northerly turn, and the channel has developed a series of symmetrical meander bends. The lower 30 km of the Pánuco is incised into Holocene deltaic deposits and has a sinuosity of 1.15, an almost straight channel. This is common in lower reaches of rivers that flow through deltaic deposits. The fine-grained clayey bank sediments are sufficiently cohesive to resist erosion (Russell et al. 1936; Kolb 1963). Secondly, the lack of sand in this portion of the river, suggested by Figure 7, does not permit the construction of point bars, which are necessary to promote lateral migration (Ikeda 1989).

An additional explanation for the highly sinuous middle Panuco is the presence of a structural control (Adams 1980; Gregory and Schumm 1987; Harbor 1998). The demarcation between the upper and middle Pánuco occurs at the axis of a north-south trending anticline, the Pánuco High. The Pánuco High, first mapped by Trager (1926) in a study of the petroleum geology of the Pánuco valley, represents a neotectonic control on contemporary channel and hydrologic processes. The axis of the anticline locally increases the floodplain elevation, and may be increasing valley gradient and stream energy downstream of the axis. To accommodate a localized increase in energy downstream of the axis, meandering rivers have been shown to adjust their channel by increasing sinuosity, which reduces channel slope and increases channel resistance (Schumm and Kahn 1972; Adams 1980; Gregory and Schumm 1987; Harbor 1998). The most sinuous channel [end p. 67] segment of the Pánuco is located immediately downstream of the anticline axis, and has a sinuosity of 3.3 (Figure 8). This phenomenon has also been documented in United States Gulf Coastal Plain rives, with the most notable example being the Lower Mississippi (Fisk 1944; Adams 1980; Burnett and Schumm 1983).

While the localized increase in floodplain elevation along the Pánuco High is relatively low, subtle changes in floodplain topography may have a significant influence on the behavior of fluvial systems. Indeed, the Pánuco High appears to have diverted the confluence of the Río Topila with the Río Pánuco downstream due to the extension of the axis south of Ciudad Pánuco (see Figure 3). The Pánuco High also locally influences the flood regime of the Río Pánuco. The river is incised where it crosses the axis of the Pánuco High, resulting in the floodplain being elevated by ~10 m above the channel. The significance of this is that it reduces the frequency of overbank flooding where the river crosses the Pánuco High. In a report on the Juastecan archeology of the Tampico region, Ekholm (1944) noted the presence of the higher floodplain surface at Ciudad Pánuco. The numerous artifacts found at this site were attributed to it being suitable for supporting a high population because of a lower incidence of flooding. The effects of the Pánuco High on the flood regime is supported by recent deposits from the large flood that occurred in October 1999, as well as by personal accounts from residents living along the river. While there was overbank flooding upstream and downstream of the axis, the river did not attain an overbank stage in the vicinity of Ciudad Pánuco, on the axis of the Pánuco High.

Conclusions

This study has offered a comprehensive assessment of spatial and seasonal (temporal) behavior of several components of the Río Pánuco drainage system. The dominant precipitation mechanisms of seasonal strengthening of summer trade winds and the occurrence of tropical storm systems serve as important controls on the hydrology and transport of sediment in the Pánuco watershed. Sediment transport occurs during high discharge events, from the late summer through early fall, with very little sediment being transported during the remainder of the year. However, while the quantity of sediment transport varies seasonally with the change in discharge, the character of sediment varies downstream due to spatial variability in watershed controls. Tributary inputs of sediments disrupt the downstream trend from an exponential curve. The percent of sand transported by the Río Pánuco increases downstream of the Río Tamuin and then displays a linear downstream reduction, reflecting storage of sediment on point bars and a lack of sediment input from tributaries. Inherited geomorphic controls appear to influence channel pattern and hydrologic processes locally. Sinuosity substantially increases downstream of the axis of the Pánuco High, while the localized increase in floodplain elevation reduces flooding.

Notes

1. This storm stalled over southeastern Mexico for two weeks and resulted in record precipitation and extreme flooding.

2. Daily discharge (Q) values in cubic meters per second (cms) were averaged for each month. The pattern reflects the strong seasonal precipitation regime, dominated by the strengthening of easterly trade-winds in July and August, and tropical storms from August through October. The uniformly low values from December through March indicate that mid-latitude cyclones, nortes, are a much less important source of precipitation for the Panuco watershed.

3. The annual pattern of suspended sediment closely mirrors the discharge trend.

4. The Río Moctezuma pattern reflects the influx of sediments from tributaries, while the downstream linear trend for the Río Pánuco reflects hydraulic sorting and deposition of sediment on point bars.

5. The Río Pánuco represents a classic example of a meandering river, with an average sinuosity of 1.85. Sinuosity for 10-km intervals is plotted, with the average sinuosity for the upper, middle, and lower reaches indicated by solid lines. The average sinuosity increases considerably in the middle Panuco, downstream of the axis of the Pánuco High.

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Resumen

La investigación de las características de la linea divisoria de las aguas proporciona a la penetración en los controles en flujo de la secuencia, transporte del sedimento, y la forma del cauce para la Río Pánuco, México. El Río Pánuco es el sexto Río más grande (98,227 km²) que fluye al Golfo de México. Ha habido muy pocos estudios de los Ríos de los sistemas llanos costeros del golfo mejicano. Los datos sugieren tres resultados primaRíos. Primero, el "streamflow" y los datos suspendidos del sedimento reflejan la sincronización y la consolidación estacionales de los vientos alisios del verano y de los sistemas tropicales de la tormenta. En segundo lugar, un perfil en el sentido descendiente de la arena de los por ciento a lo largo de la Río Moctezuma y de Río Pánuco representa una anomalia a la literatura. Finalmente, la preséncia del anticline alto de Pánuco representa un control here dado en los procesos cauces e hidrológicos contemporáneos.

Palabras Clave: Río Pánuco, México, geomorfología fluvial