Little Thompson Watershed Restoration Master Plan Prepared for: Little Thompson Watershed Restoration Coalition sponsored by the Big Thompson Conservation District with support from the Colorado Water Conservation Board Prepared by: Tetra Tech, Inc. 3801 Automation Way, Suite 100 Fort Collins, CO 80525 with support from: Logan Simpson Design, Inc. Fort Collins, CO EMPSi Inc. Boulder, CO HabiTech, Inc. Laramie, WY December 2014 TABLE OF CONTENTS December 2014 3.1.2 Reach II. Little Thompson, Berthoud: Interstate 25 to Weld-Larimer County Line.................. 27 3.1.3 Reach III. Little Thompson, Berthoud: Weld-Larimer to Larimer-Boulder County Lines ......... 28 3.1.4 Reach IV. Little Thompson, Boulder County .......................................................................... 28 3.1.5 Reach V. Little Thompson, Blue Mountain ............................................................................ 28 1 INTRODUCTION .................................................................................................................................. 1 3.1.6 Reach V. North Fork, Blue Mountain ..................................................................................... 29 1.1 Authorization .................................................................................................................................. 1 3.1.7 Reach VI. Little Thompson, Pinewood Springs...................................................................... 29 1.2 Purpose and Need ......................................................................................................................... 1 3.1.8 Reach VII(a). Little Thompson, Headwaters .......................................................................... 30 1.3 Project Scope ................................................................................................................................ 2 3.1.9 Reach VII(b). West Fork, Big Elk Meadows ........................................................................... 30 1.4 Project Area Description ................................................................................................................ 2 3.1.10 Summary of Geomorphic and Overall Channel Conditions ................................................... 31 1.5 Data, Mapping and Information Procurement ................................................................................. 2 3.2 Post Flood Vegetation and Riparian Conditions ........................................................................... 31 1.6 Stakeholder Engagement ............................................................................................................... 2 3.3 Assessment of Post Flood Conditions for Fish and Aquatic Life ................................................... 31 1.7 Acknowledgements ........................................................................................................................ 3 3.4 Bridge and Culvert Assessments ................................................................................................. 32 ................................................................................................................................................................. 4 3.5 Loss of Homes and Buildings ....................................................................................................... 32 2 WATERSHED DESCRIPTION AND EVALUATION .............................................................................. 7 4 RESTORATION RECOMMENDATIONS AND CONCEPTUAL DESIGN STRATEGIES .................... 33 2.1 Geomorphic Setting ....................................................................................................................... 7 4.1 Restoration Recommendations .................................................................................................... 33 2.2 Hydrology ...................................................................................................................................... 7 4.1.1 Reach I. Little Thompson, Milliken/Johnstown: Weld County to Interstate 25 ........................ 33 2.2.1 Background ............................................................................................................................ 7 4.1.2 Reach II. Little Thompson, Berthoud: Interstate 25 to Weld-Larimer County Line.................. 33 2.2.2 September 2013 Flood ......................................................................................................... 12 4.1.3 Reach III. Little Thompson, Berthoud: Weld-Larimer to Larimer-Boulder County Lines ......... 34 2.2.3 Stream Gage Data ................................................................................................................ 12 4.1.4 Reach IV. Little Thompson, Boulder County .......................................................................... 34 2.2.4 Existing Studies .................................................................................................................... 17 4.1.5 Reach V. Little Thompson, Blue Mountain ............................................................................ 34 2.2.5 FEMA Studies and Effective 100-year Discharges ................................................................ 17 4.1.6 Reach V. North Fork, Blue Mountain ..................................................................................... 34 2.2.6 Future Studies ...................................................................................................................... 17 4.1.7 Reach VI. Little Thompson, Pinewood Springs...................................................................... 34 2.2.7 Reservoirs ............................................................................................................................ 17 4.1.8 Reach VII. Little Thompson, Headwaters .............................................................................. 35 2.2.8 Diversions ............................................................................................................................. 18 4.1.9 Reach VII. West Fork, Big Elk Meadows ............................................................................... 35 2.2.9 Water Supply ........................................................................................................................ 18 4.2 Conceptual Design Strategies ...................................................................................................... 35 2.3 Hydraulics and Extent of Flooding ................................................................................................ 18 4.2.1 Create/Refine Low-flow Channel ........................................................................................... 35 2.4 Vegetation and Wetlands ............................................................................................................. 18 4.2.2 Excavate and Grade Low-flow Channel (capacity) ................................................................ 36 2.4.1 Overview ............................................................................................................................... 18 4.2.3 Bank Restoration .................................................................................................................. 37 2.4.2 Pre-flood Conditions ............................................................................................................. 18 4.2.4 Floodplain Grading and Stabilization ..................................................................................... 38 2.4.3 Pre-Flood Sensitive Species ................................................................................................. 21 4.2.5 Land Reclamation and Fill of Overbanks ............................................................................... 39 2.5 Fish and Aquatic Life ................................................................................................................... 22 4.2.6 Riparian Regeneration .......................................................................................................... 39 3 POST FLOOD SITE ASSESSMENTS ................................................................................................ 25 4.2.7 Active Revegetation .............................................................................................................. 40 3.1 Geomorphologic and Overall Channel Conditions ........................................................................ 25 4.2.8 Fish Habitat ........................................................................................................................... 40 3.1.1 Reach I. Little Thompson, Milliken/Johnstown: Weld County to Interstate 25 ........................ 27 4.2.9 Debris Clean-up .................................................................................................................... 40 LITTLE THOMPSON WATERSHED RESTORATION MASTER PLAN Page i TABLE OF CONTENTS December 2014 4.2.10 Ponds ................................................................................................................................... 40 List of Figures 4.2.11 Culvert and/or Bridge Replacement ...................................................................................... 41 4.2.12 Water Supply ........................................................................................................................ 41 Figure 1.1. Little Thompson River in a typical canyon-bound reach. ....................................................... 1 4.2.13 Resiliency Planning..............................................................................................................