Unknown column Array field list

Code:
*model*



class M_belajar extends CI_Model

{

 

 function save($datasensor)

 {

 $this->db->insert('tbl_water_level', $datasensor);

 return TRUE;

 }



 function river()

 {

 

 $query = $this->db->select('river_id')->from('tbl_activation')->order_by('activation_id', 'DESC')->limit('1')->get();



    return $query->row()->river_id;

 }

 

*controller*



public function sensor(){

 $this->load->model('m_belajar');



 

 if (isset($_GET['data'])) {

 

 $panjang = $this->input->get('data');

 

 $river_id['river'] = $this->m_belajar->river();

 print_r($river_id);





 $datasensor = array('water_distance' => $panjang, 'water_time' => time(), 'river_id' => $river_id );