Deciphering the relative importance of fluvial and tidal processes in the fluvial-marine transition

摘要

1.1 INTRODUCTION The interaction of tidal and river currents is a fundamental attribute of all rivers that empty into a marine basin, provided the basin is not tideless (as is essentially the case in the Mediterranean Sea and much of the Arctic Ocean; Candela, 1991; Kowalik and Proshutinski, 1994). The length of the zone of interaction is a complex function of several variables, including the coastal-plain gradient, the tidal range at the coast, and the fluvial discharge, with depth of the river playing a secondary role. A decrease in gradient causes an increase in the distance landward of the coast that tidal action can be expected, all else being equal: for example, a halving of the slope doubles the extent of tidal penetration (Fig. 1.1). An increase in the tidal range also causes tidal action to penetrate farther inland, whereas an increase in fluvial discharge decreases the tidal penetration, again all else being equal (Dyer, 1997; Nichols and Biggs, 1985). Water depth, together with channel width, also influences the upstream penetration of the tidal wave through their influence on frictional attenuation of the tide. In many systems, the landward decrease in the cross-sectional area of the channel initially causes the tidal range (and tidal-current speeds) to increase landward (i.e., a "hypersynchronous" situation; Salomon and Allen, 1983) to a location referred to as the "tidal maximum" (Dalrymple and Choi, 2007). Beyond this, friction causes the tidal range to decrease to zero at the tidal limit (Godin, 1999). Because friction is higher in shallow water, shallow systems such as braided rivers are likely to have shorter tidal-penetration distances than rivers that are deep. The end result of these various factors is that large rivers, which generally flow over low-gradient coastal plains, tend to have longer tidal-penetration distances than small rivers with steeper slopes. [Larger rivers also tend to have stronger tidal currents at their mouth because of the larger tidal prism (i.e., the tidal water flux past a point on each half tidal cycle) that is caused by the longer distance of tidal penetration (cf. Dalrymple, 2010).] The longest tidal-penetration distance documented is in the Amazon River, in which a tidal influence is detectable up to 800 km landward of the coast. However, even small to medium rivers have tidal-penetration distances of several tens to more than a hundred kilometers in low-gradient, coastal-plain settings (Dyer, 1997; A.A. Ichaso, unpublished data; Nichols and Biggs, 1985; Van den Berg et al., 2007). It follows, therefore, that a significant fraction of the coastal-zone deposits in ancient sedimentary basins should have accumulated in the tidal-fluvial transition zone.