IJERD – International Journal of Environmental and Rural Development (2012) 3-1

Research article
erd

Water Quality Improvement by Natural Meandering River
Surrounded by Woods in Agricultural Watersheds
HIROMU OKAZAWA*Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, Japan
Email:h1okazaw@nodai.ac.jp

TADAO YAMAMOTO
Research Faculty of Agriculture, Hokkaido University, Hokkaido, Japan

TAKASHI INOUE
Research Faculty of Agriculture, Hokkaido University, Hokkaido, JapanTETUAKI NAGASAWA
Research Faculty of Agriculture, Hokkaido University, Hokkaido, Japan
Received 14 December 2011

Accepted 2 April 2011(*: Corresponding Author)

Abstract Changes on nitrogen concentration and load of river water during down flow
were examined in five rivers flowing through agricultural watersheds in the Tokachi
district of eastern Hokkaido, Japan. The upper reach of each watershed is intensively
covered by cropland whereas the lower reach is covered by forest. The river form also
differs between the upper reaches, which have been artificially straightened by meander
cutoff, and the lower reaches, which remain in their natural meandering condition. Thelower the reach measured was, the lower the nitrogen concentration was, and this decreasein concentration was even more pronounced in the natural meandering sections than in thestraightened sections. The decrease of nitrogen load by natural purification in straightened
sections and natural meandering sections was examined using Lself, which was introduced
as an index of natural-purification-related nitrogen load decrease. In contrast to thechanges in concentration, nitrogen load did not decrease in the natural meandering section.
Keywords cropland, straightened river, naturally meandering river, nitrogen,self-purification

INTRODUCTION
River water contamination by agricultural nitrogen has been a serious problem in the Tokachi
district of eastern Hokkaido, Japan, where cropland cultivation is the major industry. A survey ofriver water quality in agricultural areas in the Tokachi district by Tabuchi et al., (1995), Okazawa
et al., (2009, 2010, 2011), Woli et al., (2002, 2004) revealed a strong correlation between theproportion of cropland area in a watershed and the concentration of nitrate nitrogen in the riverwater. They showed that the relationship between the two could be expressed as a linear function.
As a result, it is suggested that decreasing the area of cropland as a proportion of the watershedarea would be effective in decreasing the nitrogen concentration in the river water; however, such ameasure is not realistic because reducing the area of cropland would reduce the agriculturalproduction.
It has been known since ancient times that the water in meandering rivers (i.e. those whosechannels have not been artificially straightened) is purified by natural mechanism of selfpurification (e.g. Ajayi et al., 1984). It has been proven that, in such naturally meandering rivers,
biological, chemical and physical processes eliminate contaminants from the water during thecourse of the water’s flow through the meander sections (Heidenwag et al., 2001).
In formulating river improvement plans for areas whose main industry is cropland farming,
improvement of a river’s natural ability to purify itself can be an effective way of solving problemsⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2012) 3-1

in the water environment of the area. Preservation of natural watercourses and the use ofconstruction methods to restore streams (i.e. ecological and near-nature hydraulic engineering) canbe effective.
This study reports two years of water quality observations in watersheds in the Tokachi districtof Hokkaido. The subject area has croplands at the upper reaches and forests at the lower reaches of
its rivers, and we call such an area a “combined  agricultural area”. We clarified the self-purification
ability of the natural streams at the forested lower reaches by observing the degree of naturaldegradation of the nitrogen components emitted from the cropland field areas at the upper reaches.
METHODOLOGY
Study sites
The subject watersheds are shown in Fig.1. The survey was conducted in the watersheds of sevenrivers in the Tokachi district of Hokkaido. The basic data on each river and the watershed areshown in Table 1. The watersheds denoted as PE, PA, UR, PO and PN are those whose upper
reaches are mainly cropland fields and lower reaches are forest or pasture. Hereinafter, thesubscripts “U” and “L” will respectively denote the upper and lower reaches.
PEU, PAU, URU, POU and PNU croplands account for 70% to 86% of the total area, which ishigh percentage. In these areas, the rate of forest is 11% to 27%, and most of the forests arewindbreaks for croplands. Potatoes, legumes, wheat, feed corn and sugar beets are the main crops.
Cattle farming is also common in these upper-reach areas. Rivers have been converted to drainage
canals by straightening of the channels and concreting of the banks.
In areas at the lower reaches (PEL, PAL, URL, POL, and PNL), the rate of forest is 59% to 94%.
Cropland accounts for 6% to 41% of the areas, a large portion of which are pastures of the National
Livestock Breeding Center (NLBC). Naturally meandering river channels are preserved in theNLBC.
Watersheds BO and SY are at the NLBC. The rate of cropland is 55% and 63%, respectively,
for these watersheds, and most of the area is pasture. The streams in the two watersheds arenaturally meandering.

Fig. 1 Location maps of the investigated watershedsⒸ ISERD

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Table 1 Characteristics of investigated watershed
River

Reach

Upper
Lower
Whole area
Upper
Pankeuretoi
Lower
(PA)
Whole area
Upper
Uranenai
Lower(UR)
Whole area
Upper
PonchinLower(PO)
Whole area
Upper
Penkechin
Lower(PN)
Whole area
BokujyoBO(BO)Syuba
SY(SY)
Penkeuretoi(PE)Sampleing
point
PE1
PE2
PA 1
PA 2
UR 1
UR 2
PO 1
PO 2
PN 1
PN 2

Area
Land use (%)(km 2 ) Cropland Forest Other
5.16
83
134
2.04
35
65
0
7.20
69
28
3
4.38
84
13
3
2.04
694
0
6.42
59
39
2
2.68
85
123
4.39
27
73
0
7.07
49
50
1
12.39
70
27
3
6.46
26
73
1
18.85
54
43
3
31.04
33
64
23.82
41
59
034.863461
2Straightened Meanderingriver (km)river (km)
7.67
0.00
0.58
1.43
8.25
1.43
8.01
0.00
0.00
3.05
7.96
3.10
3.93
0.00
1.59
5.03
5.59
4.96
14.41
0.00
4.90
4.70
20.41
3.60
17.44
9.48
1.34
5.36
14.46
19.16

Total
CattleSample(km) （Head） Number
7.67
21
2.01
21
9.68
509
8.01
18
3.05
18
11.06
102
3.93
19
6.62
19
10.55
104
14.41
9
9.60
924.01
145
26.92
18
6.70
18
33.62
1,079

BO 1

1.82

55

45

0

0.00

0.96

0.96

25

SY 1

2.09

63

37

00.00

2.67

2.67

20

*Cropland consist of upland and pasuture.

Hydrological survey method
The survey period was from June to November in both 1999 and 2000. Discharge measurement andriver water sampling were done once or twice per month when the rivers were at normal waterlevel. Sampling locations for the watersheds PE, PA, UR, PO and PN are denoted by adding thesubscript  “1”  for  upper  reaches  and  “2”  for  lower  reaches.  River water samples for each sampling
location numbered between 9 and 25.
The items for water quality analysis are total nitrogen (T-N), nitrate nitrogen (NO3-N), nitrite
nitrogen (NO2-N) and ammonium nitrogen (NH4-N). Ion chromatography was used to analyze
NO3-N and NO2-N, and a technique recommended by the JIS was used to analyze T-N and NH4-N.
RESULTS AND DISCUSSION
River discharge: upper vs. lower reaches
The mean and standard deviation (S.D.) of the river discharge measured at each observation point
are shown in Fig.2. The river discharge values at PE, PA, UR, PO and PN are higher for the lowerreach points than for the upper-reach points (significances, P<0.01). This reveals that in thesewatersheds, large amounts of inflow are supplied from areas at the lower reaches of the river.
Comparison of upper-reach vs. lower-reach points in terms of nitrogen concentration
Fig.3 shows nitrogen components measured at each observation point. NO3-N accounts for 60% to
80% of the T-N at the observation points on the rivers in areas PE, PA, UR, PN and PO. Therefore,
the high proportion of NO3-N in T-N in the river waters in areas PE, PA, UR, PO and PN are
attributed to the high proportion of cropland fields in those areas.
Fig.4 shows the mean and standard deviation (S.D.) of T-N in river water sampled at eachobservation point.
At BO and SY, T-N concentrations are 0.50 mg/L and 0.53 mg/L respectively, which arelower than those of other areas. The values are lower than 1 mg/L, being this value theenvironmental standard of Japan, based on which it was determined that the water environment ofthe areas is good. The watersheds of BO and SY consist mainly of forest and pasture.
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IJERD – International Journal of Environmental and Rural Development (2012) 3-1Fig. 2 Mean and standard deviation (error bar) of river discharge for each investigated point
Statistically significant differences (P<0.01) were observed between discharge at upper-reachpoint and at lower-reach point in the watershed  “PE,  PA,  UR,  PO  and  PN”

Fig. 3 Percentage of nitrogen components for each investigated point

Fig. 4 Mean and standard deviation (error bar) of T-N concentration for each investigatedpoint
Statistically significant differences (P<0.01) were observed between nitrogen concentration at upper-reach point
and at lower-reach point in the watershed  “PE,  PA,  UR,  PO  and  PN”
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Nitrogen fertilizers are not used in forest or on pasture, and the water flowing from such areas
contains only small amounts of nitrogen, which accounts for the low nitrogen concentration in theriver water. However, in rivers in areas PE, PA, UR, PO and PN, nitrogen contamination exceeding
the T-N environmental standard of 1 mg/L is observed.
When T-N concentrations for the upper and lower observation points of areas PE, PA, UR, POand PN are compared, T-N concentrations for the lower-reach points are found to be lower thanthose for the upper-reach points. For each river, the difference in T-N concentration between theupper-reach point and the lower-reach point is significant (P<0.01). The above findings clarify thatwater high in nitrogen flows from cropland field areas at the upper reaches through rivers to thelower reaches. It was also clarified that the nitrogen concentration of the river water from theupper-reach areas decreases while flowing through the naturally meandering rivers in the forested
areas at the lower reaches.
Nitrogen load balance at the lower reaches
The nitrogen balance at the lower reaches is shown in Fig.5. Equations (1) and (2) for nitrogenbalance are also shown below. By using these two equations, the self-purification ability (Lself ) ofriver at the lower reaches was evaluated.

Fig. 5 Concept of nitrogen balance in the combined agricultural area
(The  watershed  “PE,  PA,  UR,  PO  and  PN”)
L2＝L1＋L0＋Lself∴ Lself ＝ L2−L1−L0 ＝ (C2･Q2) － (C1･Q1) －[(Q2－Q1)･CS]

(1)
(2)

L1 : Load at the upper reaches (g/s)
L2 : Load at the upper plus lower reaches (g/s)
L0 : Load from the areas at the lower reaches (g/s)
Lself : Self-purification and self-contamination of nitrogen at the river of lower reaches (g/s)
C1 : Nitrogen concentration at the upper-reach point (mg/L)
C2 : Nitrogen concentration at the lower-reach point (mg/L)
CS : Nitrogen concentration in water sampled at the natural river section of the watershed PA(0.75 mg/L, Table 2)
Q1 : Discharge at the upper-reach point (m3/s)
Q2 : Discharge at the lower-reach point (m3/s)
Q2－Q1 : Discharge from the lower reaches (m3/s)
When Lself<0, the nitrogen load is purified in the naturally meandering streams; when Lself>0,
the nitrogen load flows out of the naturally meandering streams. L0 was obtained by multiplying thesurplus discharge at the lower-reach point relative to the discharge at the upper-reach point (Q2-Q1)
by the mean nitrogen concentration of the water from tributaries that flow into the natural river
section (0.75 mg/L, Table 2).
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Table 2 Mean nitrogen concentration of inflow water from PA1 to PA2T-N

NO 3 -NNO 2 -N

NH 4 -N

Mean

0.75

0.03

0.03

0.00

S.D.

0.40

0.04

0.02

0.00

n =12,〔mg/L〕
2As shown in equation (1), L2 consists of L1，L0，and Lself. Fig. 6 shows the mean and standard
deviation of L1, L0, and Lself . Lself for each river was smaller than the load from the upper reach (L1),which means that the total nitrogen load from the upper reaches cannot be totally purified at thelower reaches. The mean Lself for rivers in watersheds UR and PN showed negative values, whichconfirms that the natural sections of these rivers have high purification ability. In all of thesurveyed rivers, the standard deviation of Lself is greater than the mean Lself, because of which Lself
can take a positive or a negative value. In the naturally meandering sections of the surveyed rivers,
natural purification at the lower reaches and nitrogen inflow from land at the lower reaches canoccur. Natural purification occurs, but the reduction in nitrogen load is thought to be small.

L1 : Load at the upper reaches (g/s)
L0 : Load from the areas at the lower reaches (g/s)
Lself : Self-purification and self-contamination of nitrogen at the river of lower reaches (g/s)
If Lself is a positive value (+), the nitrogen load flows out of the naturally meandering streams.
If Lself is a negative value (−),  the nitrogen load is purified in the naturally meandering streams.

Fig. 6 L1, L0, Lself of mean and standard deviation (error bar) for each river
CONCLUSION
The following was clarified in this study.
1. Water contaminated with nitrogen at high concentrations from the cropland fields in the areas at
the upper reaches flows into the natural river sections at the lower reaches.
2. Nitrogen concentration in the river water decreases during the course of the water flowing down
to the lower reaches. This is attributed to dilution by abundant inflow of water with low nitrogenconcentration from areas at the lower reaches.
3. In a watershed  where  land  use  is  categorized  as  “combined  agriculture”  consisting  of  croplandfields at the upper-reach areas and forest at the lower-reach areas with naturally meandering rivers,Ⓒ ISERD

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it was suggested that the nitrogen load supplied from the upper reaches to the lower reaches may bedecreased  by  the  river’s  self-purification ability. It is clarified that nitrogen load is also generated innaturally meandering river sections.
The above findings suggest that naturally meandering rivers are able to achieve selfpurification and self-contamination. Future studies should clarify the self-purification mechanismsof naturally meandering rivers. By clarifying the conditions that foster a river’s self-purification
ability, it is possible to enhance that ability by establishing engineering technologies for conserving
river water.
ACKNOWLEDGEMENTS
This research was supported by Grant-in-Aid for Young Scientists (B) from the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT), Japan. We would like to express ourgratitude to Prof. Dr. Ysushi Takeuchi (Tokyo University of Agriculture) and Mr. Yasushi Sato(Shizuoka prefecture). The authors are also grateful for the research assistance provided by thestudents of the laboratory of Land improvement and management, Graduate school of agriculture,
Hokkaido University.
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