Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M 4 , derived from the most important astronomic component in the Amazon estuary, M 2 , is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M 2 and S 2 , can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while going upstream in the estuary, reaching a maximum and then slightly decaying.

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Since it switched from a macrophyte-dominated state to a turbid, algal-dominated state in 1947, Lake Apopka has developed a layer of flocculent sediments with characteristics of fluid mud covering most of the lakebed and averaging 47 cm in thickness in 1996. Waves in this large, shallow lake frequently resuspend the upper portion of the fluid mud and frustrate programs designed to decrease the trophic state. We tested two hypotheses for its origin, one that the fluid mud layer represents the buildup of organic materials that had accumulated since 1947, the other that it was derived primarily by the liquefaction of the underlying macrophyte-derived consolidated sediments. We examined (1) changes in the mean depth of the lake relative to changes in fluid mud thickness; (2) 210 Pb dating of the sediments; (3) organic matter budget for the lake; (4) inorganic particle budget for the lake; and (5) chemical markers of sediments produced during the macrophyte stage (biogenic silica from sponges, long-chain n-alkanes from macrophytes, and 13 C) in the fluid mud. The evidence indicates that a major portion of the fluid mud can be attributed to the liquefaction of underlying consolidated sediments that were produced during the macrophyte stage of the lake. It follows that the fluid mud layer is less a direct consequence of eutrophication than a consequence of enhanced wave action on the lakebed following the loss of macrophyte dominance in this lake.

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