Estuaries may be either brackish or freshwater systems. true false 2. estuaries, much like wetlands, are breeding ground for many marine species as well as water fowl. true false 3. the biggest threat to the health of our estuaries is the erosion due to the constant movement of water, either tides, sea level flux, or currents. direct and indirect activities of man that contribute to pollution and deposition of sediments.