The amount of water moving down a river at a given time and place is referred to as its discharge, or flow, and is measured as a volume of water per unit time, typically cubic feet per second or cubic meters per second. The discharge at any given point in a river can be calculated as the product of the width (in ft or m) times the average depth (in ft or m) times average velocity (in ft/s or m/s).
The vast majority of rivers are known to exhibit considerable variability in flow over time because inputs from the watershed, in the form of rain events, snowmelt, groundwater seepage, etc., vary over time. Some rivers respond quickly to rainfall runoff or snowmelt, while others respond more slowly depending on the size of the watershed, steepness of the hillslopes, the ability of the soils to (at least temporarily) absorb and retain water, and the amount of storage in lakes and wetlands.
A hydrograph is a graph of discharge over time. The time period shown could be short, for example, the flow resulting from an individual rain storm, or it could be long, for example, a continuous record of flow over many decades. While numerous federal and state agencies, corporations, and individuals monitor discharge in streams throughout the country, the US Geological Survey is the chief entity charged with monitoring streamflow, maintaining over 9,000 stream gages, most of which record water discharge in 15 minute intervals and many of which also include water quality data. Visit the USGS Water Resource webpage (water.usgs.gov) and peruse the wealth of information compiled to assess water resources. Exercises utilizing these data are included below in module 3 as well as module 4.
The Figure 4 shows example hydrographs from the Logan River, near Logan, Utah for two different water years (2006 and 2012). The water year begins October 1 and ends September 30. Hydrologists often prefer to conduct analyses based on the water year rather than the calendar year to facilitate comparison of incoming precipitation and outgoing streamflow, and specifically to ensure that snow delivered in October, November, or December is accounted for in the same time period that it is likely to melt, which may be in spring or summer of the following calendar year.
The Logan River hydrograph shows a long (about 5 month) prominent peak in discharge, primarily driven by snowmelt, with many other smaller peaks superimposed (from accelerated snowmelt during warm periods or rain events). The hydrograph of the Logan River over a 50 year time period (Figure 6) shows the prominent peak from snowmelt each year, but provides little information about the smaller scale variability that is visible on the annual timescale. Note the non-linear y-axis of the plots. Such axes can be useful for visualizing detail in both high and low flow conditions, whereas the detail in low flows would not be visible on (typical) linear axes. The apparent shift in low flows circa 1970 on the Logan River was caused by removal of a water diversion upstream from the gauge. Note that there is a considerable amount of ‘noise’ (i.e., variability) in streamflow over the past 50 years. This variability is not random, but rather has some ‘structure’ to it, some of which is visibly obvious (annual peaks) and other portions that can only be quantified using advanced analytical or statistical techniques, which are beyond the scope of this course, but currently represent a vibrant facet of hydrologic research.
The temporal patterns of high and low flows are referred to collectively as a river’s flow regime. The flow regime plays a key role in regulating geomorphic processes that shape river channels and floodplains, ecological processes that govern the life history of aquatic organisms, and is a major determinant of the biodiversity found in river ecosystems. There are five components that characterize the flow regime:
River in regions with similar climate, geology, and topography tend to have similar flow regimes. For example, rivers draining high mountains, such as the Logan River, tend to have relatively infrequent, high magnitude, long duration, and predictable flood events that have a slow rate of change (Figure 6 on the previous page). Rivers in many tropical climates have similar flow regime characteristics as mountain rivers, due to predictable rainy and dry seasons. In contrast, rivers in arid regions are often characterized by high magnitude, short duration floods of low predictability and high flashiness (e.g., Figure 11 on the next page).
Within regions of similar climate, local factors such as soil type, soil depth, vegetation cover, and watershed size influence the natural flow regime. For example, watersheds with deep, permeable soils will be able to absorb more precipitation than watersheds with thin, impermeable soils, and will thus tend to have less flashy floods of lower magnitude and longer duration. Large rivers tend to be less flashy than small streams, which respond more quickly to individual precipitation events. Thus, natural flow regimes can be somewhat variable between nearby watersheds. Also, although general patterns in flow regime can be determined from watershed characteristics, yearly variation in precipitation patterns means that many years of flow monitoring will be required to fully characterize the flow regime of individual rivers.
Most large rivers are perennial, meaning they maintain flow throughout the year. However, many headwater streams or streams in arid regions sometimes run dry. A stream is considered temporary if surface flow ceases during dry periods. Temporary streams are often classified further as intermittent and ephemeral. An intermittent stream becomes seasonally dry when the groundwater table drops below the elevation of the streambed during dry periods. A spatially intermittent stream may maintain flow over some sections or surface water in deep pools even during dry periods due to locally elevated water tables or perched aquifers. An ephemeral stream only flows in direct response to precipitation such as thunderstorms. Thus, the flow variability of an intermittent stream is much more predictable than in an ephemeral stream.
In many parts of the world, such as the desert southwest, temporary streams may comprise a majority of the river network, >80% in some areas. However, even in wet regions, temporary streams at the head of river networks can account for >50% of the total stream network. Thus, river networks can be considered dynamic systems, with total miles of surface flow expanding and contracting in response to precipitation events.
Why would we still call a channel that goes dry for much of the year a stream? In other words, how can we distinguish between a temporary stream and an upland terrestrial ecosystem? In short, a stream has characteristic hydrological, geomorphological, and ecological processes. However, as with many topics in environmental science, the distinction between stream channels and uplands and between perennial streams and temporary streams is often fuzzy and scale-dependent. Individual stream channels may hold water for decades and then become dry during exceptional droughts that occur infrequently (once every 50-100 years). Similarly, small gullies on hillsides may flow only a few days of the year and may transport sediment but not be resident to aquatic life. Are such systems part of the river network?
A channel is generally classified as a stream based on the occurrence of several processes including Hydrological Processes, Geomorphological Processes, and Ecological Processes.
A proper stream generally consists of concentrated, channelized flow, even if it only carries water for a few days of the year. In contrast, an upland system may have surface water flow, but the flow is more akin to sheet flow and typically not concentrated into channels.
A stream channel is an area of rapid conveyance of sediment and dissolved constituents during periods of flow. However, not all sediment can be transported during all flows, and this provides a mechanism and particular pattern of sediment sorting that is a hallmark of stream channels not found in terrestrial systems.
A stream channel supports populations of aquatic organisms such as fish and insects. In contrast, upland systems do not provide even temporary habitat for aquatic organisms. Even when stream channels go dry on the surface, fish and other organisms can survive in isolated pools of water or in isolated areas of flow such as springs and perched aquifers.
Many organisms can survive in the bed of a stream channel even if the surface is dry, due to hyporheic flow, which is water that flows in the sediments of a stream channel beneath the surface.
Even if aquatic organisms do not persist in stream channels year-round, temporary flooding can provide productive systems and isolation from predators, favorable for reproduction and development of young organisms, which can then migrate to perennial rivers as the stream dries.
While it can be very informative to study hydrographs and the other flow metrics described above, often an important question often asked about rivers is ‘what percentage of time does flow exceed (or not exceed) a given value (e.g., 100 cfs)?’ It might be important to answer that question to determine the percentage of time when the flow is too low to support a particular fish species. Or it may be important to know what percentage of time the river exceeds a certain value known to cause flood damage. The proportion of time any given flow is exceeded can be determined by generating a flow duration curve. Figure 21 shows the flow duration curve for the hydrograph shown in Figure 21 (2006 water year) as well as the three subsequent years. You can immediately see that the mid and lower flows (exceeded about 40% (or 0.4) of the year) are relatively similar in each year, but the larger flows exhibit quite a bit of variability. In 2007 the highest flow of the year was only a bit over 400 cfs, while it was over 1500 cfs in 2006. The flow that was exceeded 20% of the time (0.2 on the x-axis) was approximately 450 cfs in 2005, but only 200 cfs in 2007.
Note that this plot provides detailed information on different parts of the flow duration curve depending on whether you use linear or log scales for the x or y axes (see example from the Stilliguamish River, Washington below in Figures 22-25).
Flow duration curves can be made for a given river over two different time periods to illustrate if/how the range of flows has changed over time. For example, Figure 27 shows flow duration curves for the Le Sueur River in southern Minnesota for two different time periods (1950-1970 in blue, 1990-2010 in red). Note that in these plots the fraction of year exceeded is labeled as ‘exceedance probability’. These two terms are interchangeable, both being computed as:
Where Ep is the exceedance probability or the fraction of the year that a given flow is exceeded, R is the rank, and n is the total number of values (365 if you are using daily-averaged flow values for a non-leap year). High flows (toward the left side of each plot) and low flows (toward the right side of each plot) appear not to have changed in the Elk and Whetstone rivers. In the Blue Earth River, low flows (exceeded more than 85% of the time) have not changed much, but mid-range and high flows all appear to have increased. In the Le Sueur River, the full range of flows appears to have increased. Note that the y-axis is plotted on a log scale, so even the modest difference between the two curves represents a significant increase in high flows (e.g., those that are only exceeded 5-10% of the time). The Root River, in southeastern Minnesota, has experienced significant increases in high and low flows within the past two decades, see example above.
1. What percentage of an average river network is made up of temporary streams:
(a) 0%
(b) 100%
(c) 10%
(d) 50%
ANSWER: d. 50%
2. What percentage of an average river network is made up of temporary streams:
(a) 0%
(b) 100%
(c) 10%
(d) 50%
ANSWER: b. 0.25
3. Given your answer to the previous question, how many days of the year was flow of the Logan River above 400 cfs in 2006?
(a) 37
(b) 91
(c) 256
(d) 329
ANSWER: b. 91
4. In Figure 21, what fraction of the year did flow of the Logan River exceed 400 cfs in 2007? Click to see Figure 21. [4]
(a) 0.01
(b) 0.1
(c) 0.9
(d) 0.99
ANSWER: a. 0.01
5. Given your answer to the previous question, how many days of the year was flow of the Logan River above 400 cfs in 2007?
(a) 4
(b) 37
(c) 329
(d) 361
ANSWER: a. 4
6. According to Figure 27, how much did the median (i.e., 50% exceedance) flow change in the Le Sueur River between the two time periods represented. Click to see Figure 27 [5]
(a) by a factor of 0.5
(b) by a factor of 2
(c) by a factor of 3.5
(d) by a factor of 10
ANSWER: c. by a factor of 3.5