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Hydrology and Hydraulics

The Boise River used to move across the valley from bench to bench, changing its course with spring floods and the forces of nature that resulted in the geomorphology characteristic of a large gravel bed river. Building of the three large dams upstream of Boise and development of the floodplain and river banks have resulted in a river that is constrained within its banks and the flow is managed to meet water rights requirements. The Boise River and its watershed no longer have many attributes of the river of the 1800s. Though it is changed from the historical patterns, the water flowing in the channel has properties that can be described and measured and modeled – hydrology and hydraulics.

Hydrology is the study of the amount of water that over time moves through the watershed and eventually is discharged into the river. A hydrograph is a way to plot the variation in discharge with respect to time, according to the USGS. Discharge is the volume of water flowing past a location per a unit of time, usually measured in cubic feet per second. Boise River flows are tracked by the USGS at a gage near the Glenwood Bridge. http://waterdata.usgs.gov/usa/nwis/uv?site_no=13206000. The discharge in cfs is reported numerous times a day and a daily hydrograph and historical flow information are reported on the website. The gage is operated by the USGS in cooperation with the US Army Corps of Engineers and the US Bureau of Reclamation.

A typical hydrograph for a river fed by run off from mountains is for the peak flow to be measured in spring when melting snow causes high runoff. The fall and winter flows usually are relatively stable flat lines on the graph. Because of the dams upstream of Boise, flow levels in the river no longer follow the natural hydrograph of high peak flows in the spring during run off from the mountains and a gradual decrease to relatively stable winter flows. For a river managed for irrigation, the hydrograph peak flow is lower, to reduce flooding and to hold water for summer irrigation needs. The summer discharge is higher than a natural flow because water for irrigation flows down the river until diverted by the many irrigation entities. Winter flows in the managed river are lower than the natural base flow.

Hydraulics is the applied science and engineering related to the mechanical properties of liquids. Hydraulics are the forces of moving water on the river banks and man-made structures, in short, the work done by the river. The hydraulics of the river are changed by channelization and bank hardening and other modifications to the historical channel.

One of the on going resource challenges faced by BPR is caused by the interaction of hydrology and hydraulics – bank erosion. Erosion affects the riparian area and Greenbelt. Erosion is caused by shifting gravel bars and sedimentation, which can direct flow at river banks. The flow level of the river is raised and lowered, called ramping, depending on irrigation demand and the need to evacuate space in the reservoirs for run off. Rapidly ramping down flows cause bank instability. The US Bureau of Reclamation (USBR) and the US Army Corps of Engineers (USACE) jointly manage the three storage reservoirs upstream of Boise. Those agencies are aware of the problems caused by rapid ramping rates and try not to increase or decrease flows by more than 500 cfs in one day.

Another major cause of bank erosion is from people trampling vegetation and creating paths and river access in the riparian zone. Erosion affects water quality, aquatic and terrestrial habitat, river aesthetics and over time threatens bank stability. Instability of banks may affect structures near the river and can result in significant property damage.

Once a year, representatives of BPR and Boise City Public Works inventory the river banks to determine which sites need treatments to stem erosion or to strengthen banks. The sites with most damage or most potential for encroachment into the Greenbelt are given highest priority. Bioengineering techniques, e.g., root wads, willow bundles, are preferred methods of bank stabilization and reclamation. All of the plant materials used in revegetation are native; willow cuttings are taken from other places along the Boise River. Some erosion damage can be averted by constructing properly designed, in-stream rock barbs to direct the main flow away from a vulnerable river bank. One to two sites are selected for treatment each year. Crews from BPR do the work between Nov. 1 and about March 15, after receiving all necessary permits.

The modified flows have been detrimental to some riparian species, such as the native black cottonwood, and have been advantageous for summer recreation such as tubing and play waves at diversions and the Boise River Recreation Park (BRRP).

There are eight irrigation diversions in the river reach covered by this plan. Only the Thurman Mill weir and diversion have been modified to be more compatible with recreation. A new diversion was constructed in conjunction with development of the BRRP. Other weirs and diversions, all private property owned by irrigation entities, present varying degrees of excitement and hazard to river recreationists. Recommendations in the Recreation section of the plan are for additional river recreation features, safe portages and better access for canoeists. BPR recognizes the private property rights of diversion owners and has been working to build relationships with irrigation entities to find opportunities where recreational uses and delivery of water and maintenance of private property can be achieved.

At times in the past, the Boise River has been almost dry in the winter because water rights in the reservoirs are being filled and no minimum stream flow has been designated for the river. An agreement between the US Bureau of Reclamation and the Idaho Department of Fish and Game (IDFG) makes water available for winter flows to provide streamflow maintenance from storage. When the winter flows were increased from 150 to 240 cfs in the 1980s, IDFG reported that there was a dramatic improvement in the quantity and quality of fish and wildlife habitats, particularly over wintering fish habitat.

Ponds along the Greenbelt that are now visual and recreational amenities were once sites of gravel extraction. Some of the ponds are separated from the river by a narrow strip of land often topped by the Greenbelt. These ponds are vulnerable to pit capture if in high water the river breaches the narrow strip of land and flows into the pond. When a pit is captured by the river a great deal of sediment is flushed into the river and the river may change course to a new channel through the pond. Pit capture of the ponds in the city may put park and other infrastructure at risk.

The last big flood on the Boise River was in 1943 and was the impetus for building Lucky Peak Dam. Flooding is part of the natural cycle of rivers and provides beneficial functions to a river and its floodplain. BPR land along the river can provide space for floodwater. Open space and parks are sustainable land uses in flood hazard areas. There are opportunities on BPR managed lands to reconnect the river and the floodplain. This has multiple values including attenuation of high water, restoring riparian areas, increasing fish and wildlife habitat.A flood inundation application developed by the National Weather Service, with the help of the City of Boise and other agencies, models how varying levels of flow in the Boise River would cause flooding in Boise. The modeled impact of flooding from the Boise River is at:


The ATEC report, Boise River Channel Assessment: City of Boise, Ada County, Idaho, is in Appendix A of the 1999 plan. The report identifies specific sites with erosion problems, offers potential solutions and provides a ranking of sites based on hydrological risk. The information has been used by the Boise Department of Public Works more than BPR largely because of its technical nature and identification of sites not in the jurisdiction of BPR. This study will not be updated.

Hydrology and Hydraulics 2013 Recommendations

·  When the opportunity arises to repair an irrigation weir and diversion, BP R should be prepared to identify multiple benefits from the proposed project, i.e., increase diversion efficiency and improve public safety, improve recreational experience and prevent bank erosion.

·  BPR, working with other city departments, should investigate the feasibility and cost of obtaining water rights to supplement winter stream maintenance flows and to meet other aesthetic, ecological and recreational goals.

·  Coordinate with IDFG, the USBR, and the USACE about river flows to maximize recreation, public safety, fisheries and environmental values and to maintain irrigation uses.

·  Develop a best practices and operation manuals specific to Boise River sites, e.g. no mow zones, bank stabilization, revegetation.

·  Maintain a list of approved and completed river bank projects in a geospatial database.

·  Pit capture potential will be part of each pond assessment (see Recreation section, Recreation Demand recommendations)

·  Assess opportunities for flood mitigation on BPR land.

Hydrology and Hydraulics

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