Effects of Deforestation on River Velocity

Research by Courtney Randazzo and Megan Mach
University of Washington Tacoma

|Abstract| |Introduction| |Materials and Methods| |Results| |Discussion| |Graphs| |Works Cited| |Main Page|


    The dense tropical forests of Costa Rica provide pristine habitat for a countless array of diverse species.  Much of the land in Costa Rica is protected as either national parks or private reserves.  Deforestation, however, still poses one of the largest threats to habitat preservation.  The inconsistent state of the forest surrounding the Quebrada River, located in Mastatal, has some degree of impact the riparian zone at the river’s edge. To determine how the surrounding vegetation may affect river dynamics, two sample sites along the river were closely observed. Data was collected during various precipitation conditions.  The most notable change in river dynamics was during the heavy rainfall of a flash flood.  Surface runoff was significant and notable differences were observed within minutes of the onset of the rain.

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    The tropics of Costa Rica are among the most biology diverse areas of the world.  The dense forests provide pristine habitat for the local flora and fauna; much of this land is protected as either public parks or private reserves.  Deforestation, however, is still one of the largest threats to habitat preservation in Costa Rica, which ultimately threatens environmental conditions world-wide.  Individual watersheds have experienced different degrees of loss based on deforestation rates in their vicinity.  Direct and indirect effects of deforestation may have a lasting impression on the habitat and stability of the watershed.  Land erosion, the result of losing root structures, is among the most damaging.  The river systems throughout the la congreja watershed in Mastatal, are subject to extreme variation in differing precipitation patterns.  To determine what impact, if any, deforestation has on the river dynamics of the Quebrada River, flow, depth and width were monitored during the wet season of 2004.

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Materials and Methods

    The riparian zone of the Quebrada Chires varies significantly over the length of the river.  To determine how the surrounding vegetation may affect river dynamics, two sample sites were selected.  One site was at the base of three agricultural fields which had been clear-cut and served as grazing lands for cattle.  The other site was the control site, and was surrounded by secondary rainforest. The control site was approximately 1km up-river from agricultural site.  Each sampling zone was 12 meters in length and varied in width based on precipitation conditions.  Considerations were made when the two sites were selected and every attempt was made to choose comparable conditions for that length of the river (i.e. distance from rapids, bends in river, etc).

    All data was collected three times in varying precipitation conditions.  During the rainy season, rains generally occur daily in the early afternoon.  Over the course of two weeks, we measured river conditions when (1) rains had occurred the previous day (i.e. the last 24 hours), (2) no precipitation had occurred in the previous 24 hours and (3) during heavy rainfall when precipitation had begun approximately one half hour prior to data collection.

River depth and width

    The depth of the river was measured in three places at each site.  (The “top” sampling area was the area furthest up river.) The first depth cross-section was taken at the uppermost portion of the 12 meter sampling zone.  The second was 6 meters down-river and the third was at the base of the 12 meter stretch.  The depth was measured every 0.5 meters to the nearest hundredth of a cm.  The width of the river was also calculated during the time the depth was measured at each of the three cross-sections. During the period of heavy rainfall, the depth of the river was measured only at the uppermost stretch of the 12 meter sampling zone.

River flow

    The velocity of the river flow was also measured during the three varying precipitation conditions.  At each site, we measured the time it took a floating object (ping pong ball) to travel the length of the 12 meter sampling zone.  This process was repeated five times at each site, in each condition.  Each trial was timed by two observers, and the average time was taken for each of the five trials.  In the event of severe variation (+/- 30 seconds) an additional five trials were conducted and averaged into the times measured, making a total of ten trials for that site during that particular precipitation condition.

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   River dynamics varied significantly during each of the precipitation conditions.  The most notable variation in conditions was during the heavy rains.  Surface runoff was significant and notable differences were observed within twenty minutes of the onset of the precipitation.  The rate of flow and the depth of the rivers showed drastic increase in a relatively short amount of time.


River depth and width

    The forest site experienced a higher rate of change during the heavy rains than the agriculture site.  The average depth of the river increased from 44.23cm (no rains in last 24 hours) and 43.27cm (rains in the last 24 hours) to 50.58cm (during heavy rains) (Figure 1).  The site further down river was the agriculture site and also experienced a severe change.  The variation in depth ranged from 32.25cm (rains in the last 24 hours) and 32.71 (no rains in the last 24 hours) to 35.55cm (during heavy rains) (Figure 2), however only a short period of time had passed between the time the initial reading was collected and the time the heavy rains began.  For comparison purposes, the average rate of depth increase was calculated for each of the sites.  The forest site experienced an increase at a rate of 7.3 cm per hour.  The agricultural site, 1 km down river, experienced an increase in river depth at a rate of 19.8 cm per hour.


    The width of the river also showed substantial variation.  At both sites, the river was widest during the heavy rainfall, as hypothesized.  The riparian zone was submerged along most of the river’s edge during this time.  The average width of the river at the forest site during times with no current precipitation was 4.7m.  The width increased by 0.47m to 5.17m in width, less than an hour after the rains began (Figure 1).  The agricultural site had an average width of 4.325m during time with no current precipitation, however the width expanded to 5m only a few minutes after the heavy rains began (Figure 2).


River flow

    In addition to observing the depth and width of the river in both locations, the river flow was also measured.  The velocity of the river increased substantially in a short period of time.  The forest site flow was shortest during periods of rain in the last 24 hours.  The floating ball method experienced a travel time of 99 seconds over the distance of 12 meters.  Travel time during times when no rain had occurred in the last 24 hours was 87 seconds.  During heavy rains, travel time for the same distance was reduced to 38 seconds.  The velocity of the river changed from 23.87ft/min to 27.15ft/min and finally increased to 62.16ft/min (Figure 3).


    An increase in river velocity was also observed at the agriculture site.  Since the time between precipitation conditions was less, the rate of increase the river was experiencing at the time of sampling was also less than if it had been taken at the time of the upper site.  The travel time during conditions of heavy rains in the last 24 hours was 88 seconds.  Travel time for conditions of no rain in the last 24 hours was 64, and finally, travel time of the river during periods of heavy rain decreased to 59 seconds.  The velocity rates of the river increased from 26.84ft/min to 36.91ft/min to 40.04ft/min, respectively. During the periods of heavy rainfall, the forest site data indicates a more substantial variation.  The agricultural site, however, actually experienced a greater rate of change when considered over the longer period of time. 


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    With a limited number of researchers doing a large amount of data collection it is impossible to accurately compare the forested site (Figure 1) vs. the agriculture site (Figure 2). The two sites would have to be tested simultaneously to get accurate data to compare which site is rising and running faster, which would have been vastly more accurate then taking a measurement, walking 20 minutes, and taking the next measurement. As it were, the data collected shows the speed at which the rainfall runs through the soils and into the catchment, but not which receives a greater quantity of water in a shorter amount of time.\

    The rainforest plays an important role in soil and watershed protection. The high rainfall regimes of the humid tropics mean that exposed soil is particularly liable to leaching of mineral nutrients and to erosion (Groombridge 2000). The placement of permanent plants in the soil provides a network of roots to “catch” the nutrients before they run out of the soil. When pasture is the main feature of the system, the soil is bare allowing the abundant rainfall to percolate through the soil. With living roots, such as secondary and primary rainforest, much of the water is absorbed into the plant tissue and thus, does not simply wash through the soil (Vandermeer 1995). This runoff could, in theory, be detected by the speed at which the river rises.

    The water running to each site in this system varies in both color and temperature. The forested site (Figure 1) has a water runoff temperature equal to that of the river. This water is also a distinctly different color from that coming off the agriculture site, it has a grey cloudy color, not too much sediment is accompanying the runoff. The pasture site, in comparison is at least a degree higher in temperature and the color of the soil was a deep brown-red color. Removal of forest cover modifies the rate of runoff from catchment slopes and also the density of particles carried in the drainage system (Groombridge 2002).

    What are the larger effects this deforested land will have on the area’s biodiversity? The sedimentation increase can radically change the physical environment of species restricted to particular conditions of depth, light penetration and velocity. The bared soil, with no roots to slow the water runoff, now is losing nutrients it once held as well as any herbicides, pesticides, or fertilizers that were being used to help the pasture grow. This runoff can also interfere with respiration in fish and other gill-breathing organisms further downstream (Groombridge 2002). With the increase in erosion there is a direct effect on human populations as well. Land owners begin losing their river banks when no roots are present to hold the soils, roads begin to wash out as water runoff causes soils to loosen and slide down the farmed hills. These pastures and plantations, which were once primary rainforest, are now having difficulty keeping nutrients in their acidic soils. Even if nutrients are added to the soil they will be utilized relatively inefficiently because of the acidity, and then they will be washed out of the system because of its low storage capacity (Vandermeer 1995).

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Figure 1 - Depth and width of river at forest site

Figure 2 - Depth and width of river at agriculture site

Figure 3 - River velocity at the three precipitation patterns


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Works Cited

Groombridge, B and Jenkins, MD (2000). Global Biodiversity: Earths Living Resources in the 21st Century. UNEP-World Conservation Monitoring Center. World Conservation Press. Cambridge, UK.


Groombridge, B and Jenkins, MD (2002). World Atlas of Biodiversity. UNEP-World Conservation Monitoring Center. University of California Press. Berkley, USA.


Vandermeer, John. (1995). The Ecological Basis of Alternative Agriculture. Annual Review of Ecology and Systematics 26: 201-224.


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