To describe changes in freshwater inflows to restore or conserve estuarine health, proper river and it’s associated wetland’s function, environmental flow definitions must be determined and developed (Acreman et al. 2004). In Texas, Senate Bill 3 recommended developing flow standards using an environmental flow regime, which is a schedule of the flow quantities that are satisfactory to maintaining a sound ecological environment for the persistence of key aquatic habitats (Brandes 2009a). Therefore, flow standards should mimic natural flow patterns to maintain ecological health, which is a new freshwater management strategy (Poff et al. 1997). A sound ecological environment should support native species, key habitats, key features of the natural flow regime needed for species life cycles, and sustain key ecosystem services (Brandes 2009a). Analyzing changes in freshwater inflows and the responses from other environmental components in estuarine ecosystems can provide the information necessary to create flow standards. The general methods to determine flow standards are management strategies that can be described and repeated.
Environmental Flows Defined
Environmental flows describe the hydrological properties of water flows including the quality, quantity, timing, and amount necessary to maintain estuarine ecosystem functions (Montagna et al. 2013). Healthy estuarine systems carry out functions and processes that result in biomass. Proper water allocation upstream using different management strategies increases the chance an estuarine system will continue to function properly and benefit humans.
Management strategies geared at maintaining an estuarine system try to establish environmental flows for that system. The environmental flows proposed for an estuarine system can mimic natural flow patterns or use other parameters such as estimates of the minimum flows required by that system (TCEQ 2009).
Anthropogenic and climatic influences can alter the historic flows of streams and rivers. Dams, diversions, and droughts are examples of influences that may change the inflows received by an estuary. Creating environmental flow schedules and limits through management strategies help restore and conserve estuarine systems.
The Hydrologic Cycle
The hydrological cycle begins with the evaporation of water from oceans surface. This moist air rises and begins to cool; the cooling water condenses and forms clouds. Then water from the clouds falls to the Earth as precipitation. The precipitation may infiltrate the ground and be incorporated as ground water and or it may become runoff. Some runoff evaporates from the ground surface and some flows into streams and/or rivers back into the ocean where cycle begins again. Groundwater penetrates the surface and eventually enters back into the streams and rivers or back into the atmosphere through transpiration and again the cycle continues.
Freshwater inflows are flows from rivers to estuaries. It is important to make the distinction between freshwater inflows and instream and outstream flows. Instream flows come primarily through runoff of the land, which flows, into streams and rivers. Outflows are the flows from estuaries to the coastal ocean.
Available freshwater is a very limited resource in comparison all water on Earth. About 97% of all water on Earth is in the oceans, the other 3% is freshwater that is mostly stored in glaciers and unavailable for human use. Less than one percent of the world’s freshwater is in streams and rivers. The hydrological cycle is important for determining the sources of freshwater inflows and the anthropogenic influences on those flows for any given estuarine area. Hydrology also plays a role in freshwater quantity, quality, and tidal connections.
Sources of Freshwater Inflow
Available freshwater is a very limited resource in comparison of all of the water on Earth. About 97% of all water on Earth is in the oceans, the other 3% is freshwater that is mostly stored in glaciers and unavailable for human use (USGS 2013b). Less than one percent of the world’s freshwater is in streams and rivers where it is available for human use (USGS 2013b). Freshwater use must be appropriately managed because there is such a small amount in streams and rivers. To properly manage fresh water, the sources of fresh water are important to identify because they affect the amount, timing, and frequency of fresh water (Montagna et al. 2013).
Rivers are constantly flowing and changing patterns. The changes in rivers and streams can be influenced by climate or human activities. Water in river seeks the lowest point of gravity, flowing downstream until it reaches this point. The drainage slopes that instream flows follow are called watersheds.
Watersheds are drainage basins where runoff from precipitation or springs is channeled. The extent of the watershed depends on the area and topography of the land draining to the same discharge point (USGS 2013b). Inland watersheds are areas where instream flow drains into discharge points where it either pools in lakes and ponds, and evaporation occurs, or it infiltrates the soil and becomes incorporated into ground water (USGS 2013b). Coastal watersheds may include estuaries where rivers and streams flow into the oceans and mixing occurs. Again, this water is referred to as freshwater inflows. The sources of freshwater inflows begin mostly upstream and include rainfall, river flow, runoff, groundwater, and return flows. Rainfall is a freshwater source that can occur directly on the estuary. The sources of freshwater influence the amount, timing, frequency, and duration of the freshwater inflows into the estuary (Montagna et al. 2013).
River and stream flow
Stream and river flow is dynamic in that it is continuously changing temporally and spatially. The changes in flow are primarily influenced by rainfall runoff within the particular watershed (USGS 2013c). Rainfall causes rivers to rise even if the rainfall occurs very far upland of the estuary’s watershed and eventually the water drains into the estuary (USGS 2013c). The extent and depth of the estuary influences the size of the river. A larger river will have a larger outflow point and a smaller river will have a smaller outflow point. In this way, rainfall events cause varying effects on smaller and larger estuaries. A rainfall event on a small watershed may cause the river to rise and fall rapidly with a much larger base flow than normal (USGS 2013c). In a larger watershed, a rainfall event may cause the river to rise slowly over a longer time period and flooding may last for a number of days before all of the water drains into the estuary (USGS 2013c).
The rate of flows reaching an estuary will vary depending on the amount of rainfall upstream as well as the characteristics of that stream and of that estuary. Some estuaries have a direct river source and are called river-dominated estuaries and some have indirect sources from multiple streams and runoff points and are called lagoons (Montagan et al. 2013). Major estuaries have both direct and indirect sources of flows.
River and stream flows can be studied using USGS method of using a hydrograph (USGS 2013a). A hydrograph is a chart that shows the river stage, the height of the water above an arbitrary altitude, and the streamflow, amount of water often calculated in cubic feet per second, and can also show other parameters including rainfall and water quality data (USGS 2013a).
Average annual rainfall plays a large role in the climate of a particular region. For example, a tropical rainforest such as the Amazon receives a higher mean annual rainfall, around 100 in/yr, than say the Sahara desert, which is around 5 in/y (ACEER 2001;Thinkquest 1998). Thus, annual rainfall can give indications of the kind of ecosystem that will be present. Drought resistant plants such as cacti would not be found growing on rainforest soil. That said, many estuaries that are considered “healthy” are characterized as a dynamic system that allows them to be productive by continually creating biomass and carry out life processes with the freshwater inflows they receive directly or indirectly from rainfall in the climatic region in which they exist. Changes in timing or amount of freshwater inflows into an estuary can disrupt balance of the ecosystem (Montagna et al. 2013). Therefore, understanding the frequency, timing, and magnitude of rainfall events is important for managing estuaries (Montagna 2013).
Precipitation falls directly on the estuary or is incorporated further upstream in the river. Precipitation on a stream or river changes the flow characteristics of that stream or river. USGS has gauges located at different points of several rivers that take continuous data and measure baseflow (USGS 2013a). Baseflow condition is the flow rate that occurs on average. When a rainfall event occurs, a gauge can measure the additionally resulting flow that occurs from precipitation events.
Rainfall can be plotted on a hydrograph along with streamflow. The graph provides information on the amount of rainfall and the effect by the corresponding rise in streamflow level. Rainfall for any given area can be calculated in gallons using a USGS rainfall calculator (USGS).
USGS rainfall calculator:
Streams and rivers also receive freshwater from runoff of the Earth’s surface. Some precipitation falling on land is collected by vegetation and returns through transpiration, is evaporated, or infiltrates the soil and becomes groundwater (USGS 2013d). The rest of the precipitation that falls on land hits saturated or impervious ground and flows downhill (USGS 2013d). Weather, physical geography, and topographic features of the land affect surface runoff. Water may flow from small creeks into larger creeks, streams, rivers, and often along the coast, into an estuary. Many times runoff collects the organisms, nutrients and sediments upon which it flows and these materials are suspended in the flowing water. These materials often settle out once the flow of water slows creating mud flats or sand bars in estuaries
As human population continues to increase so does the demand for fresh water. As demand for fresh water by humans increases, so does the removal of fresh water from streams, rivers, and underground water systems before it reaches the coast. Population growth also leads to more urbanization and development projects. Often natural landscapes are replaced with impervious surfaces, or surfaces that cannot be penetrated, including highways, roads, and buildings (USGS 2013d). Surface runoff cannot infiltrate the soil or become incorporated into groundwater. The rate of the flow may increase or decrease depending on what surface it flows over (USGS 2013d). For example concrete increases the flow of runoff while vegetation can slow the flow rate. Impervious surfaces such as concrete increase the rate of flow into drainage systems and streams. On impervious surfaces, the surface runoff from snow melts or storm events flows into streams at a higher volume and over a shortened amount of time.
The changes the frequency, duration, and extent of flows and can cause adverse effects including flooding of streams and rivers (USGS 2013d). Floods cause overflows of streambeds and often time the flooded water evaporates upstream before reaches the coast (USGS 2013d). Changes in surface runoff caused by altered landscapes decreases the amount and change of the physical characteristics of freshwater inflows into an estuary.
Groundwater is a large source of the fresh water that flows into streams and rivers. Water bottling manufacturers and agricultural practices require large amounts of groundwater everyday. About 30% of all fresh water on Earth is made up of groundwater (USGS 2013d). Groundwater’s significant role in contributing to freshwater flows in streams and rivers makes it important for estuaries.
Groundwater infiltrates the Earth’s surface seeking the Earth’s core, influenced by gravity (USGS 2013d). Water tends to continuously move vertically. If the water cannot penetrate any further vertically, its movement becomes more horizontal. Once water peculates into the ground, some travels close to the land surface in the water table and quickly emerges as discharge into streams and rivers (USGS 2013d). The water table is the point underground where the soil is saturated.
The ability of the water to reach greater depths depends on soil characteristics. The water underground fills the porous spaces between substrate particles. Larger particle sizes of materials such as rock hold more water then smaller particles sizes of materials such as clay and sand. Also, time plays a role in the movement of groundwater. Water closer to the surface moves back into the environment at a higher rate, whereas water further below the surface may take tens to thousands of years to move back into the ecosystem. Groundwater flow into streams and rivers contributes to the freshwater inflows reaching an estuary.
Water that returns to surface or ground water after human use is collectively called return flow. Return flows have been utilized for human use including sewage, washing, industrial uses, irrigation, and many others (USGS 2013d). Return flows are a source of freshwater inflows into estuaries. Return flows often contain high amounts of nutrients, sediments, and organisms. The effects of return flows vary but are often negative. Nutrient loading in estuaries is largely attributed to return flows (Montagna 2013). Treated wastewater can return to surface or groundwater in different ways. One of the ways wastewater is returned is by a user to a septic system that is on the users’ property directly to the groundwater system (USGS 2013d). Rural areas commonly use this method. Another way wastewater is returned by larger communities or cities is from treatment facility systems (USGS 2013d). Users release their wastewater into sewage networks that reach the treatment facility and are treated and released back into the system.
Industrial water can also be returned to surface or groundwater systems by using nondomestic on-site wastewater treatment systems to treat the water and return it to ground or surface water systems (USGS 2013d). Industrial is water that has been used to assist in manufacturing of steel, paper, chemicals, and many other commodities (USGS 2013d).
Agricultural returns are flows that have been used for irrigation and are not absorbed by plants or evaporated and enter groundwater or streams and rivers. Agricultural returns often contain nutrients and chemicals used in fertilizers and pesticides. The harmful chemicals and excess nutrients can cause negative effects in an estuarine system including harmful algal blooms.
Losses of Freshwater
Less than one percent of the total fresh water on Earth is available for human use in streams and lakes (UN Water 2013). The amount of water available today is the same amount that was available 2,000 years ago but the world’s population then was merely three percent of what it is today, making the demand and pressure on the freshwater supply increasingly strained (Lane et al. 2003). Available fresh water is also sometimes removed or lost by evaporation and human’s dams and diversions.
Evaporation is a natural process that removes freshwater from the Earth’s surface including streams and rivers, transforming liquid water into water vapor. Increased evaporation over an estuary and decreased freshwater inflows decreases the water in that estuary and increases the salt content. Salt does not evaporate and therefore increases in content in estuaries with high evaporation. Species adapted to a certain salinity range are further pressured when evaporation is high, which occurs during droughts and high temperatures.
Diversions from Anthropogenic Activities
Since the 1960s dewatering, or removal of water from streams and rivers, has doubled and around 60% of the Earth’s runoff is captured(MEA 2005). The removal of freshwater from rivers and streams for anthropogenic use before it reaches the coast is having a negative impact on many coastal estuaries. Half of the world’s major cities are within 50 km of the coast, and coastal populations are 2.6 times denser than those further inland (Crossland et al., 2005). Technological advances in the collection of freshwater is continually improving, further straining available freshwater resources. Removal of freshwater is altering three kinds of environmental flows including instream flow, freshwater flow within rivers and streams, inflow, freshwater flow into estuaries from rivers, and outflow, water flow from a river or estuary into the sea (Palmer et al. 2008).
Instream Flow Regime
Instream flow regime is the stream or river’s freshwater variability necessary to maintain or restore an estuary. The components of an instream flow regime intended to support a sound ecological environment include subsistence flows, base flows, high flow pulses, and overbanks flows and the timing, frequency, duration, and extent of each (Brandes et al. 2009a).
For information regarding the timing, frequency, duration, and extent of flows, refer to page 3 of the following online publication from Senate Bill 3 Science Advisory Committee for Environmental Flows:
Total Surface Inflows Reaching an Estuary
The Texas Water Development Board (TWDB) has established a method for calculating the total freshwater inflows reaching an estuary using United States Geological Survey (USGS) stream gauges placed upstream of 7 estuarine systems in Texas: Sabine-Neches Estuary (Sabine Lake), Trinity-San Jacinto Estuary (Galveston Bay), Lavaca-Tres Palacios Estuary (Matagorda Bay), Guadalupe Estuary (San Antonio Bay), Mission- Aransas Estuary (Aransas Bay), and Nueces Estuary (Corpus Christi Bay) from as far back as 1941.
The figure below shows sites where data was collected. The Texas coast is an ideal study area because the inflow balance decreases from northeast to southwest creating a gradient for estuarine comparison. The major estuaries in red are those that are bar-built estuaries, meaning they have parallel adjacent sandbars, islands or peninsulas.
**The Information below for total surface inflows was taken directly from the TWDB website (http://midgewater.twdb.texas.gov/bays_estuaries/hydrologypage.html):
“Total flow from drainage basin runoff is found by summing flows originating in both gaged and ungaged watersheds. Gaged flows are obtained from USGS streamflow records. Ungaged runoff is the sum of i) computed runoff, using a rainfall-runoff simulation model, based on precipitation over the watershed, ii) flow diverted from streams by municipal, industrial, agricultural, and other users, and iii) unconsumed flow returned to streams” (TWDB 2012).
An example of the spatial placement of USGS gages is shown below:
Resources: Inflow Summaries
Basin Wide Inflow summaries are also available online through the Texas Water Development Website:
Tides and Residence Times
In an estuary, freshwater inflows mix with seawater. Mixing is affected by the amount of seawater in the estuarine system. The tides and the volume of the receiving estuary govern the volume of seawater. The periodic rise and fall of the surface of the sea along the coast that are driven by the gravitational pull of the moon and of the sun are called tides (Sumich 1996). The Earth has large continents and is shaped spherically but without continents, there would be two high and low tides each lunar day (Sumich 1996). The continents interfere with the westward movement of the tidal bulges causing intricate tidal patterns within each ocean basin that different from one another (Sumich 1996).
Tidal changes are important to the coastal plants and animals that live on the boundary between land and sea in the “intertidal zone.” Tides occur in different cycles depending on the region and the time of the year. Diurnal tides occur when there is one high and one low tide a day. Semidiurnal tides occur when the rise and fall of the tides takes place twice each day. Semidiurnal mixed tides are where there are unequal tidal heights each day. Tidal levels also vary depending on what region they occur. Some tidal ranges can be small and some can be large. Tidal ranges fall into one of three categories: if the tidal range is less than 1 meter, it is microtidal; if the tidal range is above 1 meter and below 3 meters, it is mesotidal; if the tidal range is greater than 3 meters, it is macrotidal (Montagna et al. 2013).
Different freshwater inflows and tides create variations of circulation patterns in estuaries. Estuaries may be categorized as salt-wedge, well-mixed and partially mixed estuaries depending on the driving force that mixes the estuary. When river flow drives mixing in estuaries, the freshwater flows over seawater causing stratification and is categorized as a salt-wedge estuary. Wind drives the mixing in a well-mixed estuary and there is a salinity gradient that increases from the river to the estuary. Tides drive the mixing in a partially mixed estuary causing some stratification of bottom saltwater and top freshwater with gradient variation at different areas of the estuary. For example, the Gulf of Mexico has one high and one low tide, or diurnal, the U.S. West coast typically has mixed semidiurnal tides, the U.S. East coast has a more semidiurnal pattern (Montagna et al. 2013).
Flow dynamics can be understood by determining the rate at which water enters and leaves an estuary or the flushing time. The flushing time equals the volume of the estuary divided by the flow of water leaving the estuary. The flushing rate is the controlling factor of almost all estuarine processes. The flushing time can provide information on how estuaries function including the flushing time for nutrients, wastes, sediment, and organic matter, the effects of mixing, and information on many other processes (Montagna et al. 2013). Tidal mixing near shore allows for the removal of pollutants from estuaries and the circulation of nutrients. Tidal movement also transports floating organisms and vegetation to and from breeding areas to deeper water offshore.
Measuring tides and water levels
Understanding tidal fluctuations in an estuary can give indications of the relationships and life cycles of many estuarine resources as well as the flow dynamics of the system.
The National Oceanic and Atmospheric Administration (NOAA) is an ocean service agency that has been measuring, describing, and predicting tides along the coast of the U.S. since the early 1800s (NOAA 2013). A branch of NOAA that continues to record and publicize water level data is the Center for Operational Oceanographic Products and Services, CO-OPS (NOAA 2013).
The current system used by the agency uses technology that is more accurate, has advanced acoustics and electronics, records many parameters, and transmits recorded data hourly via satellites (NOAA 2013). A recorder sends an audio transmission down a half-inch-wide sounding tube and measures the time it takes for the echo to reach the surface of the water to accurately measure tidal heights (NOAA 2013). Some of the parameters measured include wind speed and direction, water current speed and direction, and air and water temperature. Raw data recorded by NOAA’s agency branches is available over the Internet (NOAA 2013).
Resources for available data:
Raw data recorded by NOAA’s agency branches is available over the Internet.
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