Coastal and subcoastal floodplain lake

Coastal and subcoastal floodplain lake – Hydrology

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• Hydrology
• Geomorphology
• Fauna
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A wealth of factors determines water quality parameters in a lake. One factor that has a large impact is whether a floodplain lake is reset or recharged by flood waters and how frequently that occurs.

waterbodies that are recharged by floodwaters retain most of the water that flows into them. Hence they also retain most of the sediment that flows in, regardless of whether or not it settles. However, sediment loading is limited due to low inflow volumes, loading in this case being simply equivalent to the water volume multiplied by the suspended sediment concentration. These sites often contain poorly sorted, consolidated bottom sediments.

In contrast, much of the water that flows into waterbodies that are reset by floods will overflow into other parts of the floodplain and/or returns to the river when the flood subsides. Input volumes are much higher than in similar-sized recharge sites but this results in increased sediment loading only if suspended sediments contained in the floodwaters passing through the water body can settle before the waters leave. Typically this could take hours or weeks, so virtually all particles larger than silt will settle. But most clay and colloid-sized particles will remain suspended and will therefore be carried away by flows.

Other factors being equal, waterbodies that temporarily detain large volumes of slowly flowing floodwater tend to accumulate the largest quantities of fine silty benthic sediment. Moreover, water residence times are highest in deep permanent sections of the water body so the average size of the sediment particles that reach the bottom are significantly smaller. Sites of this kind can contain thick (meter-deep) soft muddy bottom sediments.

Floodplain lake hydrology

The hydrology of floodplain lakes is related to a site’s topography, substrate type and position within the catchment. Climatic factors (especially those related to rainfall) also play a significant role in the regularity, seasonality, duration and amount of water entering and leaving the wetland.

Water inputs

Floodplain wetlands can be flushed with water from overbank flow, streams leading directly to the site, sheet flow across the floodplain, rainfall, run-off from the local watershed, hyporheic flow and from other groundwater sources, or a combination of these sources.

If groundwater table levels and substrate permeability permit, this can lead to more constant systems. This wetland habitat type can play a role in groundwater recharge/discharge.

An important point to note is that hyporheic flow and groundwater flow tend be clear (not turbid), compared to surface water flow.

Water outputs

Evaporation, transpiration and groundwater recharge can lead these systems to dry out, often completely.

Overbank floods move out onto the floodplain or back to the channel, connecting the wetland habitat to other wetland habitats in the floodplain. Water may also flow out to the ocean via distributary channels, depending on topography.

Hyporheic flow

Water flowing through a river can move through the sediments beneath and adjacent to the channel. This is termed the hyporheic zone, where surface water and groundwater mix. The water that flows through this zone is termed hyporheic flow, subsurface flow or base flow, and can be an important source of water for floodplain lakes and swamps.

Due to filtering by alluvial sediments (e.g. sand) hyporheic flow tends to be less turbid (clearer) than surface water flow.

Stratification

Stratification is driven by density differences within the one waterbody. Temperature has a significant effect on water density—warmer water is less dense and therefore will float on top of colder water (of the same salinity).

1. Sunlight warms the upper surface of the lake (the epilimnion), making it less dense than the water below, allowing it to float and discouraging mixing (hence inducing stratification). Only a small temperature difference is required to prevent mixing between layers, depending on the lake’s surface area, shape and wind fetch.

2. The stratification can break down through a decrease in surface temperature from cooler air temperatures that occur either at night time or seasonally, depending on the lake’s climate and other characteristics. The decreasing density of the cooling surface water, along with wind action, causes this surface water to sink and mix with the deeper water. During this time period, most of the lake water is at the same temperature, and surface and bottom waters can mix freely.

The decreasing density of the cooling surface water, along with wind action cause this surface water to sink and mix with the deeper water. During this time period, most of the lake water is at the same temperature, and surface and bottom waters can mix freely.

Stratification zones

Under certain conditions related to temperature, wind mixing, salinity, waterbody depth and vegetation cover, Queensland lakes can stratify into different layers.

The upper layer of a waterbody is a warmer (andtherefore lighter), well mixed zone called the epilimnion.

Below this is a transitional zone called the metalimnion, where temperatures rapidly change with depth. The thermocline is a horizontal plane within the metalimnion at the zone of greatest water temperature change. The metalimnion is very resistant to wind mixing.

Beneath the metalimnion, and extending to the lake bottom, is the colder, heavier, darker and relatively undisturbed hypolimnion.

More hydrology information is available in the chronology of a flooding lake.

Mixing

Stratified lakes allow very little mixing between the lake surface and the lake depths, virtually cutting off the oxygen supply to the unmixed bottom layer (the hypolimnion).

When there is little oxygen input from submerged plants (due to low light levels, for example) or when a large amount of organic matter has stimulated the growth of oxygen-consuming bacteria, the water in the unmixed bottom layer (hypolimnion) can become extremely low in oxygen (anoxic) over time.

Anoxia has a range of impacts. Not only does it make conditions harder for organisms that require oxygen for respiration to survive, it impacts on a range of chemical processes.

Anoxia and nutrients

In anoxic conditions, the nutrients phosphorus and nitrogen (in the form of ammonia) become more soluble (dissolvable) and are released from the bottom sediments into the water column. Ammonia cannot be converted to nitrate without oxygen and will therefore accumulate in anoxic water, especially under high pH (basic/alkaline) conditions. During the summer, stratified lakes can sometimes partially mix (such as with the passing of a cold front accompanied by strong winds and cold rains), allowing some of these nutrients to ‘escape’ up into the well-mixed surface waters and stimulate an algal bloom if light and temperature conditions permit. Ammonia can also directly affect animals, such as fish, that are sensitive to ammonia and are repelled by high levels in the water.

Anoxia and other trace elements

Some metals and other elements—notably iron, manganese and sulfur (as hydrogen sulfide)—also become increasingly soluble under anoxic conditions and are released from the bottom sediments, especially under acidic conditions.

These compounds cause taste and odour problems—a potentially serious concern in drinking water supply reservoirs. Additionally, high hydrogen sulfide concentrations can be lethal to many game fish as well as some zooplankton (microscopic animals that are an important fish food).

Last updated: 22 March 2013