**2.3 Pollution problems compounded by sedimentation**

Sediment mobilization from changing hydrology further complicates the pollution problems surrounding urban environments. Sediment is known to transport increased levels of many pollutants [16]. Influxes of plastics, debris and other solid waste flow freely into rivers because of direct stormwater and overland flow. Land in proximity to urban areas and watersheds with a high urban land use strongly correlate with increased concentrations of microplastics [17, 18]. Microplastics once mobilized into the river environment are concerning because of their ability to sorb/release persistent organic contaminants [19, 20]. These plastics also act to transport and provide surfaces for the growth of microbiological pathogens [21]. This complicates efforts to reduce the bacterial loading we find in these systems and heightens the concern for disease. Finally, the transport of these microplastics into the oceans from urban river drainage is very concerning and problematic [19, 22]. This problem must be controlled first in urban watersheds to provide any hope of reducing the impact in our oceans.

Pathogens (bacteria, protozoans, viruses) easily flow through the urban river environment entering from stormwater, wastewater and overflowing or leaks from sanitary sewer systems. Of these sources, stormwater generates the greatest impairment to urban rivers because of water volume [23] and concurrently is the greatest concern for disease outbreak. Using climate and epidemiological records, Rose et al. [24] found statistical evidence suggesting a correlation between storm events and disease outbreak in cities. Sediment loading of river beds along with organic material provides a good environment for bacteria such as *E. coli* to survive until the next storm event re-suspends them into river water. Pachepsky and Shelton [25] found the survivorship of *E. coli* in sediments was much greater than in overlying waters. Mallin et al. [26] attributed continual bacterial contamination from a sewage spill to release from underlying sediments well after levels depleted in overlying water. This creates the concern that urban rivers harbor extensive beds of bacteria, potential pathogens and other pollutants that will be resuspended continually as sediment and plastics move through these systems.

### **2.4 Flooding and impervious surfaces**

Interwoven into all of these problems are the changes in flood periodicity and intensity. The urban drainage network influences the river flood regime from response time due to precipitation events through the ultimate magnitude of the flood [27]. The highly impervious urban watershed has a diminutive ability to minimize flooding generating ever increasing amounts of surface runoff [28].

Development acts to amplify the runoff response causing smaller and smaller precipitation events to generate larger and larger flood events. Rainfall intensity rather than duration then becomes the driving force behind urban flooding.

Researchers investigating this phenomenon began to characterize these patterns and search for solutions. Initial characterization suggested nonporous landscapes like parking lots and buildings behaved collectively as an impervious barrier to precipitation infiltration. Calculated as a percentage, increasing coverage corresponded directly with greater volume of stormwater discharge into a river without treatment. Researchers studied how these impervious surfaces operated then incorporated these ideas into a model of impervious cover [29]. The model suggests an increasing level of stream degradation corresponding to incremental thresholds of impervious surface. Increases up to 10% of impervious surface throughout a watershed cause the river to become sensitive to inputs. Between 10% and 25%, the river becomes impacted or impaired. Beyond 25% impervious cover, the river becomes non supporting of essential river functions.

Further research found that stormwater infrastructure is actually more predictive of stream degradation than percentages of impervious surfaces [30, 31]. Research suggests that the increased complexity of stormwater of pipes and drains, the greater the impact on receiving streams. Effective Impervious Surface (EIA) was developed as a better descriptor than Total Impervious Surface (TIA) when predicting river response [32]. EIA uses the connectivity of stormwater discharge directly into the river where TIA calculates only the total surface area. Schuster et al. [28] explains the problems associated with EIA. With just a 10% level of effective impervious surface (EIA), runoff production increases to the extent that 2-year intensity storm now yields the same amount of discharge to the river as a 10-year storm. This is profound because stormwater infrastructure has now fundamentally changed watershed function. Smith et al. [27] found that the five largest floods in past 74 years in Charlotte, North Carolina occurred after 1995 suggesting that the drainage density (EIA) created this response. Such conclusions are corroborated throughout the literature [33] generating concerns that urban watersheds fundamentally changed by EIA are ill equipped to protect streams and rivers from impending climate change.

#### **2.5 Stormwater infrastructure**

Most stormwater infrastructure was built around the central premise of peak attenuation. Development requires mitigation of excessive stormwater created by the impervious surface. Most often this is some form of detention pond or other structure to slow water flow into a receiving river. The theory behind these structures is to capture the newly created runoff from development, hold it in place and then later release it at rates no greater than the pre-development peak. Thus, the peak is shaved and flattened and theoretically mimics what was discharged before development. Ecologically, this theory is flawed because a new and different stormwater peak has been generated. Roesner et al. [34] reviewed why this is so damaging to the river environment. This practice exposes the stream to extended periods of flow rather than the previous slow infiltration and discharge vegetated watersheds provided. While the peak is shaved, a greater volume of surface runoff is created and receiving streams are not in equilibrium to receive it. Further, the one size fits all mentality of design ignores unique attributes of urban landscapes for expediency. Meeting only minimum regulatory requirements (usually no greater than a ten-year storm) has built a watershed landscape that is easily overwhelmed during high intensity precipitation. Intensity-duration-frequency (IDF) curves used to engineer stormwater infrastructure may have undersized the entirety of our urban landscape as climate change impacts future precipitation patterns [35].

Even more problematic, these structures condense what was once a diffuse overlay of precipitation throughout an area into a single point discharge. This has immense ramifications on the receiving stream channel. While these structures are capable of storage and attenuation of small storms, research suggests they are highly ineffective for larger storms [36]. Further, these structures are not protective of overall degradation and any protective capabilities reduce with age [37]. Stormwater infrastructure will be very problematic as we desire to improve watersheds in the future.
