**2. Urban river degradation**

Full urban development then creates a disconnect between the benefits of urban infrastructure and the environmental costs needed to sustain it. A state of evolving equilibrium or permanent disequilibrium occurs generating pressing problems that become difficult to solve. These problems are becoming well studied and documented.

#### **2.1 Erosion and sediment mobilization**

Erosion is the fastest and most prominent visual effect of urbanization. Wolman [4] is credited with some of the earliest documentation of erosion and sedimentation directly attributed to the built environment. His observations documented export of sediment from forested watersheds (< 40 tons/year/Km2 ) into agricultural (120-320 tons/year/Km2 ) to urbanized/developed (>4,000 tons/year/ Km2 ). Further research into this problem yielded estimates from active construction as high as 50,000 tons/year/Km<sup>2</sup> [5]. The impact is diluted depending on the size of the watershed but nonetheless problematic. The final observation by Wolman [4] visually described his observations. Our urban river environment is a panorama of flood debris, sand, scoured bottoms and exposed sediment. Unfortunately, this still accurately describes the condition of many urban rivers today.

Further research and understanding of this paradigm were advanced by Chin [6]. She quantified that initial urban development mobilizes sediment on the magnitude of 2-10 times the natural rates. After development subsides, lower sediment yields predominate (< 30 tons/year/Km2 ) but a new hydrology is established. A new and enlarged river channel 2 – 15 times the original size is needed to accommodate changing water volumes discharged directly to the stream. Even though the stream enlarged, erosion continued at a rate of about 0.3 meters per year [6]. Erosion will continue until the stream channel equilibrates to watershed disturbance or more likely never subsides due to a constant pattern of disequilibrium. Discolored urban rivers of brown or orange are typical during most rain events because of this problem.

Current analysis by Gregory [7] and Maklin and Lewin [8] suggest a more holistic paradigm for river change. Time has elapsed since early urbanization impacted river systems and researchers now incorporate time scales into thinking about human impact on these systems. Small scales and short time periods cannot encompass the entirety of impacts. River systems are variable and differential in response to perturbations. While some systems are resilient others are not. Some move into a sustainable pattern while other continue a disequilibrium continuously. All river systems are impacted and now researchers are working to quantify this impact.

Because precipitation is such a strong driver on these systems, as long as the natural pattern of infiltration has been disrupted we will continue to have problems. Some river systems are continually plagued by a dense blanket of eroded material while others suffer from highly erodible river banks. These problems began in the climatically benign twentieth century [8] but will need solved under changing climatic conditions and new precipitation patterns of the next century. This may prove more challenging or even catastrophic than anything we have previously faced. Rivers need to be understood from a global scale to within small reaches to encompass the entirety of change.

## **2.2 Changes in morphology and channelization**

Beyond erosion, the continuing negative impact of a new hydrology on the physical (morphology) of the river environment is quite destructive. Excessive flow is disruptive and causes scouring of the stream bed, loss of habitat, streambank incision and isolation of the floodplain from the main channel [9]. High flow increases sheer stress on river bottom material scouring and pushing it downstream in what is called bedload. This scouring removes gravel and other material in the range of 2-64 mm [10] replacing it with bars and benches from material washed from erosional sediment above [6]. During dry periods, lack of infiltrating precipitation lowering the water table reduces lateral discharge. This creates periods of low flow degrading the aquatic environment further [11]. Rivers become 'flashy' suggesting periods of very high and very low flow rather than a consistent and stable hydrological regime.

*Degradation and Improvement of Urban River Water Quality DOI: http://dx.doi.org/10.5772/intechopen.98694*

Macroinvertebrate and fish habitat suffers. The good quality sediment and riffle habitat that the 2-64 mm material created is lost eliminating the essential refuges for aquatic life. Replacement sediment bars and benches subject to similar bed sheer stress [12] constantly change making this new environment unstable. Continual stream bank erosion adds more sediment to the stream channel until isolation from terrestrial interaction is completed [13]. Food webs become disconnected as we see less biodiversity and abundances of fish, aquatic life and the woody debris these problems generate [14].

Isolation from the floodplain is very problematic. Restriction of this hyporheic exchange limits nutrient reduction, temperature regulation and pollutant removal [15]. Rivers are part of a larger and interconnected system that cannot function well in isolation. Without the purifying mechanisms and ecological connectivity of the floodplain, the river is reduced to nothing more than a water conveyance.
