Complex sentence: Estuaries form a transition zone between river environments and maritime environments and are subject to both marine influences--such as tides, waves, and the influx of saline water--and riverine influences, such as flows of fresh water and sediment.
Split into simpler sentences: Estuaries form a transition zone between river environments and maritime environments. They are subject both to marine influences-such as tides, waves, and the influx of saline water-and to riverine influences-such as flows of fresh water and sediment.